drm: Fix DSC throughput mode 0 mask definition

Commit d7cd0e05 introduced a change at DP_DSC_THROUGHPUT_MODE_0_170
which is not aligned with the spec. This commit replace 15 << 4 by 15 <<
0 for DP_DSC_THROUGHPUT_MODE_0_170 in order to make it follow the
specification.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nikola Cornij <nikola.cornij@amd.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021150345.igdye4kv35nsk4ox@outlook.office365.com