drm/amdgpu: move the amdgpu_fbdev_set_suspend() further up

This patch moves amdgpu_fbdev_set_suspend() to the beginning
of suspend sequence.

This is to ensure fbcon does not to write to the VRAM
after GPU is powerd down.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>