drm/i915: Updating plane parameters for primary plane in setplane

This unifies how the primary plane functions work with how the sprite
functions works, which allows us to reuse them to update primary plane
properties.

v2: Moving setting of plane members in the end to take care of failure cases and
not-visible cases (Matt).

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Acked-by: Matt Roper <matthew.d.roper@intel.com>
[danvet: Add a real commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>