drm/dsc: Add native 420 and 422 support to compute_rc_params

Native 420 and 422 transfer modes are new in DSC1.2

In these modes, each two pixels of a slice are treated as one
pixel, so the slice width is half as large (round down) for
the purposes of calucating the groups per line and chunk size
in bytes

In native 422 mode, each pixel has four components, so the
mux component of a group is larger by one additional mux word
and one additional component

Now that there is native 422 support, the configuration option
previously called enable422 is renamed to simple_422 to avoid
confusion

Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190221202001.28430-3-David.Francis@amd.com