drm/msm/dsi-staging: add support for custom cmd dma scheduling

Some panels take more time to post-process the video data sent
from dsi host and hence BTA acknowledgment may take more than
one line time. This will cause dsi host to miss out sync packets
and which in turn will manifest as corruption on display panel.
To fix this, dsi host should send the BTA request after some
specified lines, once vertical active area ends. This change
adds support for configuring the custom line number at which
command dma needs to be scheduled to avoid such issues.

Change-Id: I3c83310dd755881c78fed9486f81f71d1e29916e
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>