drm/amdgpu: Move MMIO flip out of spinlocked region

Prior actual  MMIO flip we need to acquire DAL mutex to guard
our target state which get modified on reset mode.
Assign page flip status before actual flip to handle
the possible race condition with interrupt.

Signed-off-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>