msm: mdss: fix possible NULL pointer dereference

Border offsets are taken from the ctl during pipe configuration
and if mixers are not associated with ctl, it might lead to NULL
pointer dereference. NULL check was missing while logging the
border offsets. Add proper checks to avoid it.

Change-Id: Ieefab77dac21e1eb1cffcfb71979c240a490041d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>