disp: msm: dp: fix register read/write delays

Currently, for every DP hardware register read/write, there
is a string comparison to determine the execution mode. This
adds up an extra delay while powering up/down which does a
large number of register reads and writes. During stress
testing and automation, this can cause an issue resulting
in failures. Remove the unnecessary delays by using common
APIs for register reads and writes. Switch these APIs only
in case of execution mode change.

Change-Id: I9403873a29b3466c606297b2aa386d0885bb2dc7
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>