drm/dsc: Define Rate Control values that do not change over configurations

DSC has some Rate Control values that remain constant
across all configurations. These are as per the DSC
standard.

v3:
* Define them in drm_dsc.h as they are
DSC constants (Manasi)
v2:
* Add DP_DSC_ prefix (Jani Nikula)

Cc: dri-devel@lists.freedesktop.org
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Gaurav K Singh <gaurav.k.singh@intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Srivatsa, Anusha <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Link: https://patchwork.freedesktop.org/patch/msgid/20181127214125.17658-4-manasi.d.navare@intel.com