drm/radeon/kms: balance asic_reset functions

First, we were calling mc_stop() at the top of the function
which turns off all MC (memory controller) clients,
then checking if the GPU is idle.  If it was idle we
returned without re-enabling the MC clients which would
lead to a blank screen, etc.  This patch checks if the
GPU is idle before calling mc_stop().

Second, if the reset failed, we were returning without
re-enabling the MC clients.  This patch re-enables
the MC clients before returning regardless of whether
the reset was successful or not.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>