drm/amd/display: drop unused function set_abm_event()

set_abm_event() is never actually used. So, drop it.

Fixes: b8fe5637 ("drm/amd/display: Refactor ABM feature")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Tom Rix <trix@redhat.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>