msm: Add 'execute' datamover callback

Based on a patch from Brent DeGraaf:

"The datamover supports channels which can be shared amongst devices.
As a result, the actual data transfer may occur some time after the
request is queued up. Some devices such as mmc host controllers
will timeout if a command is issued too far in advance of the actual
transfer, so if dma to other devices on the same channel is already
in progress or queued up, the added delay can cause pending transfers
to fail before they start. This change extends the api to allow a
user callback to be invoked just before the actual transfer takes
place, thus allowing actions directly associated with the dma
transfer, such as device commands, to be invoked with precise timing.
Without this mechanism, there is no way for a driver to realize
this timing. Also adds a user pointer to the command structure for use
by the caller to reference information that may be needed by the
callback routine for proper identification and processing associated
with that specific request. This change is necessary to fix problems
associated with excessive command timeouts and race conditions in the
mmc driver."

This patch also fixes all the callers of msm_dmov_enqueue_cmd() to
ensure their callback function is NULL.

Signed-off-by: San Mehat <san@google.com>
Cc: Brent DeGraaf <bdegraaf@quicinc.com>
Cc: Brian Swetland <swetland@google.com>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>