drm/amd/display: Delete unused functions

[WHAT]
Removing unused code

Reviewed-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Clay King <clayking@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>