drm/dsc: Modify DRM helper to return complete DSC color depth capabilities

DSC DPCD color depth register advertises its color depth capabilities
by setting each of the bits that corresponding to a specific color
depth. This patch defines those specific color depths and adds
a helper to return an array of color depth capabilities.

v2:
* Simplify the logic (Ville)

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Reviewed-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181127214125.17658-1-manasi.d.navare@intel.com