drm/amd/amdgpu/gfx_v9_4_2: Mark functions called by reference as static

drm/amd/amdgpu/gfx_v9_4_2: Mark functions called by reference as static

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

 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1008:5: warning: no previous prototype for ‘gfx_v9_4_2_query_ras_error_count’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1054:6: warning: no previous prototype for ‘gfx_v9_4_2_reset_ras_error_count’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1063:5: warning: no previous prototype for ‘gfx_v9_4_2_ras_error_inject’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1133:6: warning: no previous prototype for ‘gfx_v9_4_2_query_ras_error_status’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1143:6: warning: no previous prototype for ‘gfx_v9_4_2_reset_ras_error_status’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/gfx_v9_4_2.c:1153:6: warning: no previous prototype for ‘gfx_v9_4_2_enable_watchdog_timer’ [-Wmissing-prototypes]

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>