drm/i915: Unclutter the get_plane() functions

crtc->base.primary->fb was used everywhere. Use fb to temporarily point
there and don't forget to assign fb to its final destination at the end.

v2: Rebase on top of misc changes (mask of DSPSURF, PAGE_ALIGN)

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>