drm/amdgpu: mark symbols static where possible

We get 7 warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:1990:5: warning: no previous prototype for 'amdgpu_pre_soft_reset' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c:1548:5: warning: no previous prototype for 'amdgpu_connector_virtual_dpms' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c:1560:5: warning: no previous prototype for 'amdgpu_connector_virtual_set_property' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c:330:5: warning: no previous prototype for 'amdgpu_cs_list_validate' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/dce_virtual.c:98:6: warning: no previous prototype for 'dce_virtual_stop_mc_access' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/dce_virtual.c:130:6: warning: no previous prototype for 'dce_virtual_resume_mc_access' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/dce_virtual.c:136:6: warning: no previous prototype for 'dce_virtual_set_vga_render_state' [-Wmissing-prototypes]

In fact, all of the functions are only used in the file
in which they are declared and don't need a declaration,
but can be made static.

So this patch marks both functions with 'static'.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>