drm/i915/dsc: Add debugfs entry to validate DSC output formats

DSC_Output_Format_Sink_Support entry is added to i915_dsc_fec_support_show
to depict if sink supports DSC output formats (RGB/YCbCr420/YCbCr444).
Also, new debugfs entry is created to enforce output format. This is
required because of our driver policy. For ex. if a mode is supported
in both RGB and YCbCr420 output formats by the sink, our policy is to
try RGB first and fall back to YCbCr420, if mode cannot be shown
using RGB. So, to test other output formats like YCbCr420 or YCbCr444,
we need a debugfs entry (force_dsc_output_format) to force this
output format.

v2: -Func name changed to intel_output_format_name() (Jani N)
    -Return forced o/p format from intel_dp_output_format() (Jani N)
v3: -output_format_str[] to remain static (Jani N)

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230309062855.393087-8-suraj.kandpal@intel.com