drm/imagination: Check for NULL struct dev_pm_domain_list

drm/imagination: Check for NULL struct dev_pm_domain_list

While dev_pm_domain_detach_list() itself contains the necessary NULL check,
the access to struct dev_pm_domain_list->num_pds does not and thus faults
on devices with <=1 power domains (where the struct dev_pm_domain_list
machinery is skipped for simplicity).

This can be reproduced on AM625, which produces the following log[1]:

[   10.820056] powervr fd00000.gpu: Direct firmware load for powervr/rogue_33.15.11.3_v1.fw failed with error -2
[   10.831903] powervr fd00000.gpu: [drm] *ERROR* failed to load firmware powervr/rogue_33.15.11.3_v1.fw (err=-2)
...
[   10.844023] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018
...
[   11.090162] Call trace:
[   11.092600]  pvr_power_domains_fini+0x18/0xa0 [powervr] (P)
[   11.098218]  pvr_probe+0x100/0x14c [powervr]
[   11.102505]  platform_probe+0x5c/0xa4

Fixes: e19cc5ab347e3 ("drm/imagination: Use dev_pm_domain_attach_list()")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/r/c353fdef-9ccd-4a11-a527-ab4a792d8e70@sirena.org.uk/ [1]
Tested-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Alessio Belle <alessio.belle@imgtec.com>
Link: https://patch.msgid.link/20260227-single-domain-power-fixes-v1-1-d37ba0825f7c@imgtec.com
Signed-off-by: Matt Coster <matt.coster@imgtec.com>