disp: msm: dsi: Add a new function to get drm_panel

A new API to get drm_panel from dsi_display.
The drm_panel was defined as a member of drm_connector.
When doing connector initialize, can set drm_panel to
drm_connector. So this API is needed to get drm_panel
from dsi_dsiplay.

Change-Id: I0ec9de5a9407085048a8fef421b7b28d466085ed
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>