svga: flush buffers when mapping for reading

svga: flush buffers when mapping for reading

With host-side buffer copies (via SVGA3D_vgpu10_BufferCopy()) we have
to make sure any pending map-write operations are completed before reading
if the buffer is dirty.  Otherwise the ReadbackSubResource operation could
get stale data from the host buffer.

This allows the piglit arb_copy_buffer-subdata-sync test to pass when
we start using the SVGA3D_vgpu10_BufferCopy command.

v2: check the sbuf->dirty flag in the outer conditional, per Charmaine.

Acked-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>