drm/msm/dsi-staging: null pointer check at once after call dsi_panel_get

null pointer check at once after called dsi_panel_get function.

Signed-off-by: Chong Gu <chongu@codeaurora.org>
Change-Id: Ib36d7daf86d9f72e069b4cc0ab2d1e2c100efcb7