drm/i915/skl: Fix always true comparison in a revision id check

It's always a good idea to keep static analysis happy (also because it
prompts doing the check like I proposed :), this time smatch complains:

drivers/gpu/drm/i915/intel_ringbuffer.c:891 gen9_init_workarounds() warn:
  always true condition '((->dev->pdev->revision) >= (0)) => (0-255 >= 0)'

That's because revision is a u8. Tweak a bit the condition then.

Cc: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -957,8 +957,8 @@ static int gen9_init_workarounds(struct intel_engine_cs *ring)
 	WA_SET_BIT_MASKED(HALF_SLICE_CHICKEN3,
 			  GEN9_DISABLE_OCL_OOB_SUPPRESS_LOGIC);
 
-	if (INTEL_REVID(dev) >= SKL_REVID_A0 &&
-	    INTEL_REVID(dev) <= SKL_REVID_B0) {
+	if (INTEL_REVID(dev) == SKL_REVID_A0 ||
+	    INTEL_REVID(dev) == SKL_REVID_B0) {
 		/*
 		* WaDisableDgMirrorFixInHalfSliceChicken5:skl
 		* This is a pre-production w/a.