drm/amdgpu: Add DC feature mask to disable fractional pwm

drm/amdgpu: Add DC feature mask to disable fractional pwm

[Why]

Some LED panel drivers might not like fractional PWM. In such cases,
backlight flickering may be observed.

[How]

Add a DC feature mask to disable fractional PWM, and associate it with
the preexisting dc_config flag.

The flag is only plumbed through the dmcu firmware, so plumb it through
the driver path as well.

To disable, add the following to the linux cmdline:
amdgpu.dcfeaturemask=0x4

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204957
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Tested-by: Lukáš Krejčí <lskrejci@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>