drm/msm/dsi-staging: register panel with global registry

allow other device drivers to find this drm panel in global registry

Change-Id: I945bdbe9d8ed85dbb20f876e72687d6363c27492
(cherry picked from commit 9fd9ee68951d477aa9be580b047adcb72eff55c3)
Signed-off-by: Harpreet "Eli" Sangha <eliptus@google.com>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>