drm/i915: Use fb->format->is_yuv for the g4x+ sprite RGB vs. YUV check

g4x+ sprites have an extra cdclk limitation listed for RGB formats.
For some random reason I chose to use cpp>=4 as the check for that.
While that does actually work let's deobfuscate it by checking
for !is_yuv instead. I suspect is_yuv didn't exist way back when
I originally write the code.

Also drop the duplicate comment.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200206201204.31704-2-ville.syrjala@linux.intel.comAcked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_sprite.c

@@ -1639,8 +1639,8 @@ static int g4x_sprite_min_cdclk(const struct intel_crtc_state *crtc_state,
 	limit -= decimate;
 
 	/* -10% for RGB */
-	if (fb->format->cpp[0] >= 4)
-		limit--; /* -10% for RGB */
+	if (!fb->format->is_yuv)
+		limit--;
 
 	/*
 	 * We should also do -10% if sprite scaling is enabled