msm: mdss: Use clock safe wrapper for reading DSI hw version

Attempting to read without proper clock votes will result
in a unclocked access leading to a watchdog. Enable only
the DSI BUS clocks instead of all the DSI clocks while
reading the DSI revision register.

Change-Id: Ia83fda764d145e96d2df813379c57cf6529b1c0c
Signed-off-by: Naveen Ramaraj <nramaraj@codeaurora.org>
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>