drm/radeon/cik: Move 'vce_v2_0_enable_mgcg()'s prototype to shared header

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

 drivers/gpu/drm/radeon/vce_v2_0.c:111:6: warning: no previous prototype for ‘vce_v2_0_enable_mgcg’ [-Wmissing-prototypes]
 111 | void vce_v2_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: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/cik.c

@@ -41,6 +41,7 @@
 #include "radeon_audio.h"
 #include "radeon_ucode.h"
 #include "si.h"
+#include "vce.h"
 
 #define SH_MEM_CONFIG_GFX_DEFAULT \
 	ALIGNMENT_MODE(SH_MEM_ALIGNMENT_MODE_UNALIGNED)
@@ -129,7 +130,6 @@ MODULE_FIRMWARE("radeon/mullins_rlc.bin");
 MODULE_FIRMWARE("radeon/mullins_sdma.bin");
 
 static u32 cik_get_cu_active_bitmap(struct radeon_device *rdev, u32 se, u32 sh);
-extern void vce_v2_0_enable_mgcg(struct radeon_device *rdev, bool enable);
 static void cik_rlc_stop(struct radeon_device *rdev);
 static void cik_pcie_gen3_enable(struct radeon_device *rdev);
 static void cik_program_aspm(struct radeon_device *rdev);

drivers/gpu/drm/radeon/vce.h

@@ -30,5 +30,6 @@
 struct radeon_device;
 
 void vce_v1_0_enable_mgcg(struct radeon_device *rdev, bool enable);
+void vce_v2_0_enable_mgcg(struct radeon_device *rdev, bool enable);
 
 #endif                         /* __VCE_H__ */

drivers/gpu/drm/radeon/vce_v2_0.c

@@ -30,6 +30,7 @@
 #include "radeon.h"
 #include "radeon_asic.h"
 #include "cikd.h"
+#include "vce.h"
 
 #define VCE_V2_0_FW_SIZE	(256 * 1024)
 #define VCE_V2_0_STACK_SIZE	(64 * 1024)