drm/i915/display/dsc: Force dsc BPP

Set DSC BPP to the value forced through
debugfs. It can go from bpc to bpp-1.

v2: Use default dsc bpp when we are just
    doing force_dsc_en, use default dsc bpp
    for invalid force_dsc_bpp values. (Jani)

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720064907.9771-4-vandita.kulkarni@intel.com