drm/i915/dsc: Add debugfs entry to validate DSC fractional bpp

DSC_Sink_BPP_Precision entry is added to i915_dsc_fec_support_show
to depict sink's precision.
Also, new debugfs entry is created to enforce fractional bpp.
If Force_DSC_Fractional_BPP_en is set then while iterating over
output bpp with fractional step size we will continue if output_bpp is
computed as integer. With this approach, we will be able to validate
DSC with fractional bpp.

v2:
Add drm_modeset_unlock to new line(Suraj)

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110101020.4067342-8-ankit.k.nautiyal@intel.com