drm/mediatek: Handle drm_atomic_helper_swap_state failure

drm/mediatek: Handle drm_atomic_helper_swap_state failure

drm_atomic_helper_swap_state() will be changed to interruptible waiting
in the next few commits, so all drivers have to be changed to handling
failure.

Signed-off-by: Maarten Lankhorst <[email protected]>
Cc: CK Hu <[email protected]>
Cc: Philipp Zabel <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Philipp Zabel <[email protected]>
Reviewed-by: Sean Paul <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>