drm/msm/dsi: Create a helper to check if there is a connected device

Create a helper msm_dsi_device_connected() which checks whether we have a
device connected to the dsi host or not. This check gets messy when we
have support external bridges too. Having an inline function makes it
more legible.

For now, the check only consists of msm_dsi->panel being non-NULL. Later,
this will check if we have an external bridge or not.

This helper isn't used in dsi_connector related code as that's specific
to only when a drm_panel is connected.

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