drm/amdkfd: Validate user queue buffers

Find user queue rptr, ring buf, eop buffer and cwsr area BOs, and
check BOs are mapped on the GPU with correct size and take the BO
reference.

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