msm_fb: display: mdp4 version assigned at board file

The mdp4 version reflects mdp's capability and is referred to at
many places. Currently, it is acquired by reading mdp version
register. Since the MDP version is defined in the board file,
this patch uses that version at probe time.

Change-Id: Iae2db1fb8609cb80f3705b5e47d3fefedc15fec5
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>