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 <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/vce_v2_0.c

···
 	WREG32(VCE_CGTT_CLK_OVERRIDE, 7);
 }
 
+/*
+ * Local variable sw_cg is used for debugging purposes, in case we
+ * ran into problems with dynamic clock gating. Don't remove it.
+ */
 void vce_v2_0_enable_mgcg(struct radeon_device *rdev, bool enable)
 {
 	bool sw_cg = false;