drm/i915: s, fb->bits_per_pixel/8, pixel_size, in primary plane code

Use the pixel_size we got from drm_format_plane_cpp() instead of
fb->bits_per_pixel/8 when computing the primary plane page/linear
offsets. Avoids a few divs and makes the code more future proof
against funky pixel formats where bits_per_pixel isn't well defined.
This is what we already did in the sprite code.

Note that the relevant sprite patch was

commit ca320ac4
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Dec 19 12:14:22 2012 +0000

    drm/i915: Use pixel size for computing linear offsets into a sprite

This change was required on sprites because they support yuv formats
which have fb->bits_per_pixel undefined.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add Chris' software archeology as a note to the commit
message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2466,12 +2466,12 @@ static void i9xx_update_primary_plane(struct drm_crtc *crtc,
 	if (IS_G4X(dev))
 		dspcntr |= DISPPLANE_TRICKLE_FEED_DISABLE;
 
-	linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
+	linear_offset = y * fb->pitches[0] + x * pixel_size;
 
 	if (INTEL_INFO(dev)->gen >= 4) {
 		intel_crtc->dspaddr_offset =
 			intel_gen4_compute_page_offset(&x, &y, obj->tiling_mode,
-						       fb->bits_per_pixel / 8,
+						       pixel_size,
 						       fb->pitches[0]);
 		linear_offset -= intel_crtc->dspaddr_offset;
 	} else {
@@ -2574,10 +2574,10 @@ static void ironlake_update_primary_plane(struct drm_crtc *crtc,
 	if (!IS_HASWELL(dev) && !IS_BROADWELL(dev))
 		dspcntr |= DISPPLANE_TRICKLE_FEED_DISABLE;
 
-	linear_offset = y * fb->pitches[0] + x * (fb->bits_per_pixel / 8);
+	linear_offset = y * fb->pitches[0] + x * pixel_size;
 	intel_crtc->dspaddr_offset =
 		intel_gen4_compute_page_offset(&x, &y, obj->tiling_mode,
-					       fb->bits_per_pixel / 8,
+					       pixel_size,
 					       fb->pitches[0]);
 	linear_offset -= intel_crtc->dspaddr_offset;
 	if (to_intel_plane(crtc->primary)->rotation == BIT(DRM_ROTATE_180)) {