drm/i915: Initialise entry in intel_ppat_get() for older compilers

gcc-4.7.3 is confused by the guards inside intel_ppat_get() and reports:

drivers/gpu/drm/i915/i915_gem_gtt.c: In function ‘intel_ppat_get’:
drivers/gpu/drm/i915/i915_gem_gtt.c:3044:27: warning: ‘entry’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Forgive the compiler this once, and rearrange the code so that entry is
always initialised.

v2: Flavour with a bit of NULL (instead of ERR_PTR(-ENOSPC))

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115131705.16341-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3076,7 +3076,7 @@ const struct intel_ppat_entry *
 intel_ppat_get(struct drm_i915_private *i915, u8 value)
 {
 	struct intel_ppat *ppat = &i915->ppat;
-	struct intel_ppat_entry *entry;
+	struct intel_ppat_entry *entry = NULL;
 	unsigned int scanned, best_score;
 	int i;
 
@@ -3099,7 +3099,7 @@ intel_ppat_get(struct drm_i915_private *i915, u8 value)
 	}
 
 	if (scanned == ppat->max_entries) {
-		if (!best_score)
+		if (!entry)
 			return ERR_PTR(-ENOSPC);
 
 		kref_get(&entry->ref);