drm/i915/dsc/mtl: Add support for fractional bpp

Consider the fractional bpp while reading the qp values.

v2: Use helpers for fractional, integral bits of bits_per_pixel. (Suraj)

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110101020.4067342-6-ankit.k.nautiyal@intel.com