RKL only has five universal planes, plus a cursor.  Since the
bottom-most universal plane is considered the primary plane, set the
number of sprites available on this platform to 4.

In general, the plane capabilities of the remaining planes stay the same
as TGL.  However the NV12 Y-plane support moves down to the new top two
planes and now only the bottom three planes can be used for NV12 UV.

Bspec: 49181
Bspec: 49251
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200504225227.464666-8-matthew.d.roper@intel.com