pmdomain: arm: Fix NULL dereference on scmi_perf_domain removal

On unloading of the scmi_perf_domain module got the below splat, when in
the DT provided to the system under test the '#power-domain-cells' property
was missing. Indeed, this particular setup causes the probe to bail out
early without giving any error, which leads to the ->remove() callback gets
to run too, but without all the expected initialized structures in place.

Add a check and bail out early on remove too.

 Call trace:
  scmi_perf_domain_remove+0x28/0x70 [scmi_perf_domain]
  scmi_dev_remove+0x28/0x40 [scmi_core]
  device_remove+0x54/0x90
  device_release_driver_internal+0x1dc/0x240
  driver_detach+0x58/0xa8
  bus_remove_driver+0x78/0x108
  driver_unregister+0x38/0x70
  scmi_driver_unregister+0x28/0x180 [scmi_core]
  scmi_perf_domain_driver_exit+0x18/0xb78 [scmi_perf_domain]
  __arm64_sys_delete_module+0x1a8/0x2c0
  invoke_syscall+0x50/0x128
  el0_svc_common.constprop.0+0x48/0xf0
  do_el0_svc+0x24/0x38
  el0_svc+0x34/0xb8
  el0t_64_sync_handler+0x100/0x130
  el0t_64_sync+0x190/0x198
 Code: a90153f3 f9403c14 f9414800 955f8a05 (b9400a80)
 ---[ end trace 0000000000000000 ]---

Fixes: 2af23ceb

 ("pmdomain: arm: Add the SCMI performance domain")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240125191756.868860-1-cristian.marussi@arm.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>