drm/radeon: add header comment for clarification to vce_v2_0_enable_mgcg()

Add function header comment to make it clear that local variable sw_cg
is used for debugging and it should not be removed.

Addresses-Coverity-ID: 1198635
Cc: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>