drm/amd/display: Revert in_transfer_func_change to MED

drm/amd/display: Revert in_transfer_func_change to MED

[Why]
Last commit accidentally changed handling of in_transfer_func_change
from MED to FAST.

[How]
* Revert the line.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Dominik Kaszewski <dominik.kaszewski@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>