drm/radeon/cik: Move 'Move 'cik_sdma_*()'s prototypes to shared header

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

drivers/gpu/drm/radeon/cik_sdma.c:331:6: warning: no previous prototype for ‘cik_sdma_enable’ [-Wmissing-prototypes]
331 | void cik_sdma_enable(struct radeon_device *rdev, bool enable)
| ^~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/cik_sdma.c:528:5: warning: no previous prototype for ‘cik_sdma_resume’ [-Wmissing-prototypes]
528 | int cik_sdma_resume(struct radeon_device *rdev)
| ^~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/cik_sdma.c:557:6: warning: no previous prototype for ‘cik_sdma_fini’ [-Wmissing-prototypes]
557 | void cik_sdma_fini(struct radeon_device *rdev)
| ^~~~~~~~~~~~~

Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/cik.c

···
 MODULE_FIRMWARE("radeon/mullins_sdma.bin");
 
 static u32 cik_get_cu_active_bitmap(struct radeon_device *rdev, u32 se, u32 sh);
-extern int cik_sdma_resume(struct radeon_device *rdev);
-extern void cik_sdma_enable(struct radeon_device *rdev, bool enable);
-extern void cik_sdma_fini(struct radeon_device *rdev);
 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);

drivers/gpu/drm/radeon/cik.h

···
 u32 cik_get_csb_size(struct radeon_device *rdev);
 void cik_get_csb_buffer(struct radeon_device *rdev, volatile u32 *buffer);
 
+int cik_sdma_resume(struct radeon_device *rdev);
+void cik_sdma_enable(struct radeon_device *rdev, bool enable);
+void cik_sdma_fini(struct radeon_device *rdev);
 #endif                         /* __CIK_H__ */