drm/panfrost: Do not check for 0 return after calling platform_get_irq_byname()

It is not possible for platform_get_irq_byname() to return 0.
Use the return value from platform_get_irq_byname().

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803040401.3067484-2-ruanjinjie@huawei.com