修复amlogic s922x未接显示器时会导致内核报错的问题
[CRTC:46:meson_crtc] vblank wait timed out
WARNING: CPU: 2 PID: 265 at drivers/gpu/drm/drm_atomic_helper.c:1682 drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
CPU: 2 UID: 0 PID: 265 Comm: setfont Tainted: G C
Tainted: [C]=CRAP
pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
lr : drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
Call trace:
drm_atomic_helper_wait_for_vblanks.part.0+0x240/0x264
drm_atomic_helper_commit_tail_rpm+0x84/0xa0
commit_tail+0xa4/0x18c
drm_atomic_helper_commit+0x164/0x178
drm_atomic_commit+0xb4/0xec
drm_client_modeset_commit_atomic+0x210/0x270
drm_client_modeset_commit_locked+0x5c/0x188
drm_fb_helper_pan_display+0xb8/0x1d4
fb_pan_display+0x7c/0x120
bit_update_start+0x20/0x48
fbcon_switch+0x418/0x54c
el0t_64_sync+0x194/0x198