drm/amd/pm/powerplay/hwmgr/hwmgr: Move 'vega12_hwmgr_init()'s prototype to shared header

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

 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega12_hwmgr.c:2862:5: warning: no previous prototype for ‘vega12_hwmgr_init’ [-Wmissing-prototypes]
 2862 | int vega12_hwmgr_init(struct pp_hwmgr *hwmgr)
 | ^~~~~~~~~~~~~~~~~

Cc: Evan Quan <evan.quan@amd.com>
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/amd/pm/inc/hwmgr.h

@@ -830,5 +830,6 @@ int hwmgr_handle_task(struct pp_hwmgr *hwmgr,
 
 int smu7_init_function_pointers(struct pp_hwmgr *hwmgr);
 int smu8_init_function_pointers(struct pp_hwmgr *hwmgr);
+int vega12_hwmgr_init(struct pp_hwmgr *hwmgr);
 
 #endif /* _HWMGR_H_ */

drivers/gpu/drm/amd/pm/powerplay/hwmgr/hwmgr.c

@@ -47,7 +47,6 @@ extern const struct pp_smumgr_func smu10_smu_funcs;
 extern const struct pp_smumgr_func vega20_smu_funcs;
 
 extern int vega10_hwmgr_init(struct pp_hwmgr *hwmgr);
-extern int vega12_hwmgr_init(struct pp_hwmgr *hwmgr);
 extern int vega20_hwmgr_init(struct pp_hwmgr *hwmgr);
 extern int smu10_init_function_pointers(struct pp_hwmgr *hwmgr);