drm/amd/display: Skip modeset for front porch change

[Why]
A seamless transition between modes can be performed if the new incoming
mode has the same timing parameters as the optimized mode on a display with a
variable vtotal min/max.

Smooth video playback usecases can be enabled with this seamless transition by
switching to a new mode which has a refresh rate matching the video.

[How]
Skip full modeset if userspace requested a compatible freesync mode which only
differs in the front porch timing from the current mode.
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>