drm/msm/dsi: Fix potential NULL pointer dereference in msm_dsi_modeset_init

_dev_ is being dereferenced before it is null checked, hence there
is a potential null pointer dereference.

Fix this by moving the pointer dereference after _dev_ has been
null checked.

Fixes: d4e7f38d70ef ("drm/msm/dsi: check msm_dsi and dsi pointers before use")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>