drm/i915/dsc: Allow DSC only with fractional bpp when forced from debugfs

If force_dsc_fractional_bpp_en is set through debugfs allow DSC iff
compressed bpp is fractional. Continue if the computed compressed bpp
turns out to be a integer.

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

Signed-off-by: Swati Sharma <swati2.sharma@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/20231110101020.4067342-9-ankit.k.nautiyal@intel.com