drm/i915/dsc: Refactor dsc gen checks

Use HAS_DSC(__i915) wrapper containing runtime info of has_dsc
member. Platforms supporting dsc has this flag enabled; no need of
DISPLAY_VER() check.

Also, simplified intel_dsc_source_support() based on above changes.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221110093312.13932-1-swati2.sharma@intel.com