drm/radeon/si_dma: Move 'si_gpu_check_soft_reset()'s prototype to shared header

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

drivers/gpu/drm/radeon/si.c:3768:5: warning: no previous prototype for ‘si_gpu_check_soft_reset’ [-Wmissing-prototypes]
3768 | u32 si_gpu_check_soft_reset(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: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/si.h

···
 struct radeon_device;
 
 int si_mc_load_microcode(struct radeon_device *rdev);
+u32 si_gpu_check_soft_reset(struct radeon_device *rdev);
 
 #endif                         /* __SI_H__ */

drivers/gpu/drm/radeon/si_dma.c

···
 #include "radeon.h"
 #include "radeon_asic.h"
 #include "radeon_trace.h"
+#include "si.h"
 #include "sid.h"
-
-u32 si_gpu_check_soft_reset(struct radeon_device *rdev);
 
 /**
  * si_dma_is_lockup - Check if the DMA engine is locked up