drm/i915: Call .update_primary_plane in intel_{enable, disable}_primary_hw_plane()

Make the intel_{enable,disable}_primary_hw_plane() simply call
.update_primary_plane(), thus eliminating the rmw from these functions
which should help the poor old 830M.

Now we can also remove the .update_primary_plane() from the
.crtc_enable() hooks because we end up calling it via
intel_crtc_enable_planes()->intel_enable_primary_hw_plane().

This also has the nice benefit of making primary planes a bit closer to
the way we handle sprite planes during modesets.

v2: Just write 0 to DSPCNTR and DSPSURF/DSPADDR if the plane is (to be)
    disabled. Quicker, and more importantly avoids an oops when fb==NULL
    due to BIOS fb takeover failure.
    Pimp the commit message a bit (Matt)
v3: Drop useless primary_enabled checks when setting DISPLAY_PLANE_ENABLE

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>