amdgpu/dc/dml: Consolidate redundant CFLAGS

Use a variable for common CFLAGS instead of specifying the same flags
for every source file.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>