drm/radeon: replace vmram_mutex with mclk_lock v2

It is a rw_semaphore now and only write locked
while changing the clock. Also the lock is renamed
to better reflect what it is protecting.

v2: Keep the ttm_vm_ops on IGPs

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>