drm/amd/display: Remove unused duplicate register definition

[Why]
DCN32 uses ABM register definitions in dcn32_resource.h, remove
duplicate from dce_abm.h to avoid confusion.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>