disp: msm: dsi: add null check for panel and proper ctrl iteration

This change moves the panel null check to the beginning of the function
so panel can be used throughout the function. This change also replaces
looping through display ctrls with proper display_for_each_ctrl.

Change-Id: I0014ee7ad6d8514734f9233a1abb314e60d29b5f
Signed-off-by: Samantha Tran <samtran@codeaurora.org>