drm/msm/dsi: Refer to connected device as 'device' instead of 'panel'

We currently support only panels connected to dsi output. We're going to
also support external bridge chips now.

Change 'panel_node' to 'device_node' in the struct msm_dsi_host and
'panel_flags' to 'device_flags' in msm_dsi. This makes things sound a
bit more generic.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>