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>