drm/i915: Initialize new chv primary plane and pipe blender registers

CHV adds a bunch of new registers for primary plane size/position and
pipe blender setup. Initialize all those registers to avoid nasty
surprises. PRIMSIZE is especially important as without programming it
the outout will be garbled whenever the primary plane size would not
match what the BIOS set up.

Also program the sprite constant alpha register to disable the constant
alpha blending factor. This applies to vlv as well as chv.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>