drm/amdgpu: keep the MMU lock until the update ends v4

This is quite controversial because it adds another lock which is held during
page table updates, but I don't see much other option.

v2: allow multiple updates to be in flight at the same time
v3: simplify the patch, take the read side only once
v4: correctly fix rebase conflict

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>