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 <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>

drivers/gpu/drm/radeon/si.h

@@ -27,5 +27,6 @@
 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

@@ -25,10 +25,9 @@
 #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
  *