msm: mdss: protect mdp registers from race condition

During the pipe configuration process it was possible for multiple
thread contexts to read and write common register data, resulting in
a race condition. In order to prevent a race condition, we are now
protecting access to common registers are with reg_lock mutex.

Change-Id: I724e7870e856b3303448edbdf00f334ad38483f6
Signed-off-by: Terence Hampson <thampson@codeaurora.org>