drm/i915: Eliminate rmw from .update_primary_plane()

Move the entire DSPCNTR register setup into the .update_primary_plane()
functions. That's where it belongs anyway and it'll also help 830M which
has the extra problem that plane registers reads will return the value
latched at the last vblank, not the value that was last written.

Also move DSPPOS and DSPSIZE setup there.

v2: Don't move variable initialization to avoid churn later

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>