msm: mdss: fix potential crash when sending backlight DCS commands

If MDSS idle power collapse feature is enabled, MDSS GDSC can get
toggled when exiting idle power collapse. This may require an
explicit call to TZ to restore MDSS security configs prior to accessing
any MDSS registers, which is typically done whenever IOMMU is attached.
For panels where backlight is controlled using DCS commands, it is
possible that a backlight update could be triggered when MDSS is power
collapsed. This would result in enabling MDSS GDSC and restoring DSI
registers prior to sending the DCS commands. For such use cases, ensure
that IOMMU is attached prior to any register programming to avoid any
crashes.

Change-Id: I245d1180cfeedeb938e70b792fd6d627e86c5d14
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>