drm: Fix DSC BPP increment decoding

Starting with DPCD version 2.0 bits 6:3 of the DP_DSC_BITS_PER_PIXEL_INC
DPCD register contains the NativeYCbCr422_MAX_bpp_DELTA field, which can
be non-zero as opposed to earlier DPCD versions, hence decoding the
bit_per_pixel increment value at bits 2:0 in the same register requires
applying a mask, do so.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Fixes: 0c2287c9 ("drm/display/dp: Add helper function to get DSC bpp precision")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212161851.4007005-1-imre.deak@intel.com