drm/radeon: remove volatile qualifier

There doesn't seem to be any need to have 'ib' volatile, the code is
not even consistent with it and some places already miss it. As it is
now it's just making gcc produce worse code. If there are special
requirements for that memory, then proper primitives like memory
barriers or accessor functions should be used, but it doesn't look
like that is needed here.
While at it, change the type to match the one in radeon_ib structure.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>