drm/i915: Clean up intel_plane_atomic_check_with_state()

Rename some of the state variables in
intel_plane_atomic_check_with_state() to make it less confusing.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111170823.4441-2-ville.syrjala@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>