input: touchscreen: fix kernel crash in fb_notifier_callback function

input: touchscreen: fix kernel crash in fb_notifier_callback function

fb_event would only carry the data number in some special notify action,
other actions wouldn't carry an valid data number, and in this case
kernel would crash, logs like:

[    4.129846] Unable to handle kernel paging request at virtual address 200000000000
......
[    4.164618] Hardware name: Rockchip RK3399 Evaluation Board v1 (Chrome OS) (DT)
[    4.184624] PC is at fb_notifier_callback+0x28/0xac
[    4.189497] LR is at notifier_call_chain+0x74/0xb4
[    4.194279] pc : [<ffffffc0005e1468>] lr : [<ffffffc0000b5ba4>] pstate: 20000045
......
[    5.703780] [<ffffffc0005e1468>] fb_notifier_callback+0x28/0xac
[    5.709690] [<ffffffc0000b5ba4>] notifier_call_chain+0x74/0xb4
[    5.715504] [<ffffffc0000b5e70>] __blocking_notifier_call_chain+0x48/0x64
[    5.722280] [<ffffffc0000b5ea0>] blocking_notifier_call_chain+0x14/0x1c
[    5.728885] [<ffffffc00036fd98>] fb_notifier_call_chain+0x20/0x28
[    5.734969] [<ffffffc0003726c0>] register_framebuffer+0x218/0x250
[    5.741054] [<ffffffc0003b7598>] drm_fb_helper_initial_config+0x2f8/0x374
[    5.747832] [<ffffffc0003e056c>] rockchip_drm_fbdev_init+0xa8/0xe8
[    5.754002] [<ffffffc0003dba24>] rockchip_drm_load+0x1e4/0x25c

Change-Id: I3314315a31bbab43489fca85dabc4c6511fc9dee
Signed-off-by: Yakir Yang <ykk@rock-chips.com>