msm: sde: correct polling to use software timestamp

In polling mode, driver is using the top level busy and
error status for completion check. This is not correct
for regdma because it has separate status register and
software timestamp. This patch corrects regdma to wait
for the appropriate status register and checks for software
timestamp for timeout.

CRs-Fixed: 1096834
Change-Id: I18f8ab14be1db4d24954115b763e6ac749b71b4d
Signed-off-by: Alan Kwong <akwong@codeaurora.org>