drm/amd/display: Fix VRR cannot enable

[Why]
Sometimes the VRR cannot enable after login to the desktop.

User space may call the DRM_IOCTL_MODE_GETCONNECTOR right after
the DRM_IOCTL_MODE_RMFB.

After calling DRM_IOCTL_MODE_RMFB to remove all the frame buffer
and it will cause the driver to disable the crtc and disable the
link while calling the link_set_dpms_off().

It will cause the dpcd read failed in amdgpu_dm_update_freesync_caps()
while try to get the DP_MSA_TIMING_PAR_IGNORED capability and think
the sink side does not support VRR.

[How]
Use the dpcd_caps.allow_invalid_MSA_timing_param flag instead of
reading from dpcd directly.

dpcd_caps.allow_invalid_MSA_timing_param flag is updated during HPD.
It is safe to replace the original method.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>