drm/msm/sde: fix iounmap usage and init callflow

Fix iounmap usage to properly use devm_iounmap. Fix clearing of
pointers in failure callflows of sde_kms init.

Change-Id: Ib42134ee972c578070ac513c3e4da37faec7d0cd
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>