[SYCL]: Remove per-allocation Level Zero runtime checks (#23399)

* [SYCL] Centralize Level Zero detection in ggml_sycl_init

* use the same wording

* get back the warning

* [SYCL] Remove per-allocation getenv() for GGML_SYCL_ENABLE_LEVEL_ZERO

* bring back the comment

* move it up to make sure devices call the shots

* move the env detection early

* replace g_ggml_sycl_enable_level_zero with a direct call to .ext_oneapi_level_zero

* update the comment

* switch back to g_ggml_sycl_enable_level_zero with a sentinel

* remove the check

* Reduce the diff

* reword, move lower

* move things aroudn

* remove forward declaration if favor of a full replace

* pre-cache results of zeDeviceGetProperties

* put ggml_sycl_get_env back

* replace get_sycl_env with ggml_sycl_get_env

* add whitespace back

* Apply suggestion from @sanmai

ggml/src/ggml-sycl/common.cpp

@@ -59,7 +59,7 @@ bool gpu_has_xmx(sycl::device &dev) {
     return dev.has(sycl::aspect::ext_intel_matrix);
 }
 
-static int ggml_sycl_get_env(const char *env_name, int default_val) {
+int ggml_sycl_get_env(const char *env_name, int default_val) {
     char *user_device_string = getenv(env_name);
     int user_number = default_val;
 
@@ -86,7 +86,7 @@ int64_t downsample_sycl_global_range(int64_t accumulate_block_num, int64_t block
 
 #ifdef GGML_SYCL_SUPPORT_LEVEL_ZERO
 static bool ggml_sycl_use_level_zero_device_alloc(sycl::queue &q) {
-    return ggml_sycl_get_env("GGML_SYCL_ENABLE_LEVEL_ZERO", 1) &&
+    return g_ggml_sycl_enable_level_zero &&
         q.get_device().is_gpu() &&
         q.get_backend() == sycl::backend::ext_oneapi_level_zero;
 }
@@ -94,8 +94,6 @@ static bool ggml_sycl_use_level_zero_device_alloc(sycl::queue &q) {
 
 // Use Level Zero zeMemAllocDevice to avoid sycl::malloc_device triggering
 // DMA-buf/TTM system RAM staging in the xe kernel driver during multi-GPU inference.
-// The decision is made from the queue and runtime env because large buffers can be
-// allocated before ggml_check_sycl() initializes g_ggml_sycl_enable_level_zero.
 void * ggml_sycl_malloc_device(size_t size, sycl::queue &q) {
 #ifdef GGML_SYCL_SUPPORT_LEVEL_ZERO
     if (ggml_sycl_use_level_zero_device_alloc(q)) {