drm/amdgpu: nuke the ih reentrant lock

Interrupts on are non-reentrant on linux. This is just an ancient
leftover from radeon where irq processing was kicked of from different
places.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>