drm/radeon/si_dpm: Move 'vce_v1_0_enable_mgcg()'s prototype to shared header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/vce_v1_0.c:102:6: warning: no previous prototype for ‘vce_v1_0_enable_mgcg’ [-Wmissing-prototypes]
 102 | void vce_v1_0_enable_mgcg(struct radeon_device *rdev, bool enable)
 | ^~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>