drm/dsc: add helper for calculating rc buffer size from DPCD

drm/dsc: add helper for calculating rc buffer size from DPCD

Add a helper for calculating the rc buffer size from the DCPD offsets
DP_DSC_RC_BUF_BLK_SIZE and DP_DSC_RC_BUF_SIZE.

Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6c6aee22740fe7a8cf2b8e768bfda378850cf59a.1607429866.git.jani.nikula@intel.com