drm/i915: move I915_IDLE_ENGINES_TIMEOUT next to its only user

Declutter i915_drv.h. If there's ever a need to use this in more than
one place, we can figure out a better spot then. For now, this seems
easiest.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118131538.3558599-2-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_debugfs.c

@@ -648,6 +648,9 @@ i915_drop_caches_get(void *data, u64 *val)
 
 	return 0;
 }
+
+#define I915_IDLE_ENGINES_TIMEOUT (200) /* in ms */
+
 static int
 gt_drop_caches(struct intel_gt *gt, u64 val)
 {

drivers/gpu/drm/i915/i915_drv.h

@@ -193,8 +193,6 @@ struct i915_gem_mm {
 	u32 shrink_count;
 };
 
-#define I915_IDLE_ENGINES_TIMEOUT (200) /* in ms */
-
 #define HAS_HW_SAGV_WM(i915) (DISPLAY_VER(i915) >= 13 && !IS_DGFX(i915))
 
 struct i915_virtual_gpu {