msm: kgsl: Clear VBIF pending transactions before going to power collapse

Before VBIF can be power collapsed, the bus must be left in a good state.
This means there can be no outstanding transactions on the bus that have
not yet been returned to VBIF.  If VBIF is power collapsed while there
are still outstanding transactions, the transactions may degrade system
performance or may confuse VBIF when VBIF is powered up again,
leading to a functional failure or a hang.

The clearest way to ensure there are no outstanding transactions is to
use the VBIF halt procedure.  VBIF halt is a SW handshake using
VBIF_AXI_HALT_CTRL0 and VBIF_AI_HALT_CTRL1. Once a halt has been initiated,
VBIF will stop accepting new requests from the client interfaces,
VBIF will flush all transactions and wait for them to complete,
and VBIF will indicate that the halt has been completed.

Change-Id: I7bb38b5dea2446427fdbd6a7b8515cf72dd09e6b
Signed-off-by: Tarun Karra <tkarra@codeaurora.org>