drm/amd/display: Fix black screen with scaled modes on some eDP panels

[why]
This was a regression introduced by commit:

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

Due to the change how timing parameters were set, scaled modes would
cause a black screen on some eDP panels. Would probably apply to
other displays (i.e. even non-eDP) that only have scaled modes,
but such case is not that usual for external displays.

[how]
Pick up crtc frame dimensions when programming the timing unless
it's FreeSync video mode.

Fixes: 6f59f229 ("drm/amd/display: Skip modeset for front porch change")
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>