drm/dp: Add DP DSC DPCD receiver capability size define and missing SHIFT

This patch defines the DP DSC receiver capability size that gives
total number of DP DSC DPCD registers.
This also adds a missing #defines for DP DSC support missed in the
commit id (ab6a46ea

 "Add DPCD definitions for DP 1.4 DSC feature")

v3:
* MIN_SLICE_WIDTH = 2560 (Anusha)
* Define DP_DSC_SLICE_WIDTH_MULTIPLIER = 320
v2:
* Add SHIFT define and DECOMPRESSION_EN define missed in prev patch

Cc: dri-devel@lists.freedesktop.org
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031001923.31442-2-manasi.d.navare@intel.com