intel/common: add directory prefix to intel_gem.h include

Otherwise common/intel_gem.h may get included instead of
common/i915/intel_gem.h when building in a different directory.

Fixes: e050a00b9f ("intel/common: Move i915 files to i915 folder")
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26784>
(cherry picked from commit fbb9ce0140)

.pick_status.json

@@ -294,7 +294,7 @@
         "description": "intel/common: add directory prefix to intel_gem.h include",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "e050a00b9f4d057e93f61f79019b5cf8bcd04e49",
         "notes": null

src/intel/common/i915/intel_engine.c

@@ -25,7 +25,7 @@
 
 #include <stdlib.h>
 
-#include "intel_gem.h"
+#include "i915/intel_gem.h"
 
 static enum intel_engine_class
 i915_engine_class_to_intel(enum drm_i915_gem_engine_class i915)