drm: mali-dp: Add debugfs file for reporting internal errors

Status register contains a lot of bits for reporting internal errors
inside Mali DP. Currently, we just silently ignore all of the errors,
that doesn't help when we are investigating different bugs, especially
on the FPGA models which have a lot of constraints, so we could easily
end up in AXI or underrun errors.

Add a new file called debug that contains an aggregate of the
errors reported by the Mali DP hardware.

E.g:
[root@alarm ~]# cat /sys/kernel/debug/dri/1/debug
[DE] num_errors : 167
[DE] last_error_status  : 0x00000001
[DE] last_error_vblank : 385
[SE] num_errors : 3
[SE] last_error_status  : 0x00e23001
[SE] last_error_vblank : 201

Changes since v2:
- Add lock to protect the errors stats.
- Add possibility to reset the error stats by writing anything to the
  debug file.

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>