drm/v3d: Don't clear MMU control bits on exception

MMU exception conditions are reported in the V3D_MMU_CTRL register as
write-1-to-clear (W1C) bits. The MMU interrupt handling code clears any
exceptions, but does so by masking out any other bits and writing the
result back. There are some important control bits in that register,
including MMU_ENABLE, so a safer approach is to simply write back the
value just read unaltered.

This patch doesn't remove the cause of the apparent PTE errors, but it
does reduce the impact to just an error in the kernel log.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>