drm/msm/dsi-staging: fix dsi read for generic read commands

Current display driver only handles dsi DCS read commands, extend
the same to handle dsi generic read commands as well.

Change-Id: I856fd95c55db6702551aa791d4c5e2df98761fd7
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>