drm/i915: Move the pxp plane state computation

No real reason to have this pxp state computation in
intel_atomic_check_planes(). Just stuff it into skl_plane_check().

There was also some funny state copying being done from the
old plane state to the new plane state when the plane is anyway
disabled.

The one thing we presumably must remember to do is copy
over the decrypt state when assigning a Y plane for planar
YCbCr scanout, so that the Y plane's PLANE_SURF will get the
appropriate bit set. The force_black thing should not matter
as I'm pretty sure all that stuff is ignored for the Y plane.
I suppose this was the reason for the odd placement for the
state computation, but I see no reason to deviate from the
standard way of doing these things. This also guarantees
that we don't calculate things differently between the
linked UV and Y plane.

v2: Only do stuff for icl+ since 'force_black' depends
    on the plane CSC which is an icl+ feature

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Juston Li <juston.li@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211006235704.28894-2-ville.syrjala@linux.intel.com
Reviewed-by: Juston Li <juston.li@intel.com> #v1