drm/amd/display: Adjust functions documentation

Part of the documentation of the 'dc_process_dmub_aux_transfer_async'
function was misplaced, being put together with the
‘dc_enable_dmub_notifications’ documentation. This caused the following
warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:3757: warning:
expecting prototype for dc_process_dmub_aux_transfer_async(). Prototype
was for dc_enable_dmub_notifications() instead

This commit fixes the warning by placing the function documentations in
their right place.

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>