drm/i915: Initialise GTT MTRR to -1

Fixes a regression from 9e984bc1 (drm/i915: Don't do MTRR setup if PAT
is enabled) where we left the MTRR as 0 and so tried to free a MTRR we
did not own during unload.
Reported-and-tested-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1923,6 +1923,8 @@ static void
 i915_mtrr_setup(struct drm_i915_private *dev_priv, unsigned long base,
 		unsigned long size)
 {
+	dev_priv->mm.gtt_mtrr = -1;
+
 #if defined(CONFIG_X86_PAT)
 	if (cpu_has_pat)
 		return;