drm/amdgpu: use drm_fb helper for console_(un)lock

This patch removes the usage of console_(un)lock
by replacing drm_fb_helper_set_suspend() to
drm_fb_helper_set_suspend_unlocked() which locks and
unlocks the console instead of locking ourselves.

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>