drm/amd/display: Remove unused CFLAGS entry in DML Makefile

Missed that with a previous change that removed unused files.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>