drm/amd/display: Remove unnecessary call to REG_SEQ_SUBMIT|WAIT_DONE

[why & how]
Remove unnecessary call to REG_SEQ_SUBMIT and REG_SEQ_WAIT_DONE, since
those macros are not necessary anymore at the dpp1 set degamma. Those
are part of an old implementation.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@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>