drm/meson: fix shutdown crash when component not probed

In the Linux kernel, the following vulnerability has been resolved:

drm/meson: fix shutdown crash when component not probed

When main component is not probed, by example when the dw-hdmi module is

not loaded yet or in probe defer, the following crash appears on shutdown:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000038

...

pc : meson_drv_shutdown+0x24/0x50

lr : platform_drv_shutdown+0x20/0x30

...

Call trace:

meson_drv_shutdown+0x24/0x50

platform_drv_shutdown+0x20/0x30

device_shutdown+0x158/0x360

kernel_restart_prepare+0x38/0x48

kernel_restart+0x18/0x68

__do_sys_reboot+0x224/0x250

__arm64_sys_reboot+0x24/0x30

...

Simply check if the priv struct has been allocated before using it.