drm/i915/display: Prepare for dsc 3 stream splitter

At the moment dsc_split represents whether the dsc splitter is used
or not. With 3 DSC engines, the splitter can split into two streams
or three streams.

Instead of representing the splitter's state, it is more effective to
represent the number of DSC streams per pipe.

Replace the `dsc.dsc_split` member with `dsc.num_streams` to indicate the
number of DSC streams used per pipe. This change will implicitly
convey the splitter's operation mode.

v2: Avoid new enum for dsc split. (Suraj)
v3:
-Replace dsc_split with num_stream. (Suraj)
-Avoid extra parentheses. (Jani)
v4: Set num_streams to 1, if VDSC_JOINER not set while readout.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030041036.1238006-3-ankit.k.nautiyal@intel.com