drm/i915: Clean up pre-skl primary plane registers

drm/i915: Clean up pre-skl primary plane registers

Use REG_BIT() & co. for the pre-skl primary plane registers.
Also give everything a consistent namespace.

v2: s/DSP/DISP/ to avoid confusion (José)
    Use DISP_WIDTH rather than DISP_POS_X for DSPSIZE (José)
    Deal with gvt

Cc: José Roberto de Souza <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: José Roberto de Souza <[email protected]>