drm/i915/dsc: make DSC source support helper generic

Move intel_dp_source_supports_dsc() from intel_dp.c as
intel_dsc_source_support() in intel_vdsc.c. The DSC source support is
more about DSC than about DP, and will be needed for DP independent
code.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6c9f646090913290fb00efd46a4332421bf95930.1575974743.git.jani.nikula@intel.com