drm/amd/display: Remove unused function

The get_engine_type is never used in the code, for this reason, this
commit drops this function.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>