Fixes the following gcc with W=1:

drivers/gpu/drm/radeon/radeon_atpx_handler.c:64:6: warning: no previous prototype for ‘radeon_has_atpx’ [-Wmissing-prototypes]
   64 | bool 4(void) {
      |      ^~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:68:6: warning: no previous prototype for ‘radeon_has_atpx_dgpu_power_cntl’ [-Wmissing-prototypes]
   68 | bool radeon_has_atpx_dgpu_power_cntl(void) {
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:72:6: warning: no previous prototype for ‘radeon_is_atpx_hybrid’ [-Wmissing-prototypes]
   72 | bool radeon_is_atpx_hybrid(void) {
      |      ^~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:77:6: warning: no previous prototype for ‘radeon_atpx_dgpu_req_power_for_displays’ [-Wmissing-prototypes]
   77 | bool radeon_atpx_dgpu_req_power_for_displays(void) {
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:596:6: warning: no previous prototype for ‘radeon_register_atpx_handler’ [-Wmissing-prototypes]
  596 | void radeon_register_atpx_handler(void)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:614:6: warning: no previous prototype for ‘radeon_unregister_atpx_handler’ [-Wmissing-prototypes]
  614 | void radeon_unregister_atpx_handler(void)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/radeon/radeon_atpx_handler.c:159: warning: expecting prototype for radeon_atpx_validate_functions(). Prototype was for radeon_atpx_validate() instead

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>