drm/panfrost:fix the exception name always "UNKNOWN"

The exception_code in register is only 8 bits,So if
fault_status in panfrost_gpu_irq_handler() don't
(& 0xFF),it can't get correct exception reason.

and it's better to show all of the register value
to custom,so it's better fault_status don't (& 0xFF).

Signed-off-by: ChunyouTang <tangchunyou@icubecorp.cn>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708073407.2015-1-tangchunyou@163.com