drm/i915/dp: Avoid left shift of DSC output bpp by 4

To make way for fractional bpp support, avoid left shifting the
output_bpp by 4 in helper intel_dp_dsc_get_output_bpp.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817142459.89764-12-ankit.k.nautiyal@intel.com