drm/i915: Clean up DSC vs. not bpp handling

drm/i915: Clean up DSC vs. not bpp handling

No point in duplicating all this code when we can just
use a variable to hold the output bpp (the only thing
that differs between the two branches).

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.comk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326144903.6617-2-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>