drm/msm: initialize mode_config functions before HW init

The DRM framework in 4.14 has been updated to take special actions
if the sub-driver implements the atomic commit callback.  During the
hw init, there is a code path that leads to this check.  This can
lead to a null pointer dereference if the mode_config functions
are not initialized beforehand.

Change-Id: Ic036870c2db49ae3a086bfc0e1a959575b8e0254
Signed-off-by: Steve Cohen <cohens@codeaurora.org>