drm/msm: Defer VMA unmap for fb unpins

With the conversion to drm_gpuvm, we lost the lazy VMA cleanup, which
means that fb cleanup/unpin when pageflipping to new scanout buffers
immediately unmaps the scanout buffer.  This is costly (with tlbinv,
it can be 4-6ms for a 1080p scanout buffer, and more for higher
resolutions)!

To avoid this, introduce a vma_ref, which is incremented whenever
userspace has a GEM handle or dma-buf fd.  When unpinning if the
vm is the kms->vm we defer tearing down the VMA until the vma_ref
drops to zero.  If the buffer is still part of a flip-chain then
userspace will be holding some sort of reference to the BO, either
via a GEM handle and/or dma-buf fd.  So this avoids unmapping the VMA
when there is a strong possibility that it will be needed again.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661538/