drm/radeon|amgpu: Make fbdev emulation optional

Seems to at least compile fine, only change needed was to use
the fb_set_suspend helper.

Cc: alexander.deucher@amd.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470847958-28465-5-git-send-email-daniel.vetter@ffwll.ch