drm/amd/display/amdgpu_dm/amdgpu_dm_pp_smu: Mark local functions invoked by reference as static

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

 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:538:6: warning: no previous prototype for ‘pp_rv_set_wm_ranges’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:590:6: warning: no previous prototype for ‘pp_rv_set_pme_wa_enable’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:601:6: warning: no previous prototype for ‘pp_rv_set_active_display_count’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:614:6: warning: no previous prototype for ‘pp_rv_set_min_deep_sleep_dcfclk’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:627:6: warning: no previous prototype for ‘pp_rv_set_hard_min_dcefclk_by_freq’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_pp_smu.c:640:6: warning: no previous prototype for ‘pp_rv_set_hard_min_fclk_by_freq’ [-Wmissing-prototypes]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@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>