drm/amdgpu: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/si.c:908:5: warning: no previous prototype for 'si_pciep_rreg' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/si.c:921:6: warning: no previous prototype for 'si_pciep_wreg' [-Wmissing-prototypes]

In fact, both 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 these functions with 'static'.

Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
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>