drm/amd/display: Turn functions into static

Silence [-Wmissing-prototypes] sparse warnings from the display folder
such as:

../drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn315/dcn315_smu.c:126:5: warning: no previous prototype for ‘dcn315_smu_send_msg_with_param’ [-Wmissing-prototypes]
  126 | int dcn315_smu_send_msg_with_param(
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

v2: drop unused function dcn315_smu_set_voltage_via_phyclk() to avoid
    adding a warning (Alex)

Cc: Qingqing Zhuo (Lillian) <qingqing.zhuo@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>