drm/amdgpu: add license to umc_8_7_0_sh_mask.h

Was missing. Add it.

Fixes: 6b36fa61 ("drm/amdgpu: add umc v8_7_0 IP headers")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>