drm/radeon: drop register readback in cayman_cp_int_cntl_setup

The read is taking a considerable amount of time (about 50us on this
machine). The register does not ever hold anything other than the ring
ID that is updated in this exact function, so there is no need for
the read modify write cycle.

This chops off a big chunk of the time spent in hardirq disabled
context, as this function is called multiple times in the interrupt
handler. With this change applied radeon won't show up in the list
of the worst IRQ latency offenders anymore, where it was a regular
before.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org