msm: mdss: Remove usage of deprecated macro MDSS_MDP_REG_INTF_OFFSET

MDSS_MDP_REG_INTF_OFFSET macro is deprecated and should not be used,
instead get the interface offset from device tree.

Change-Id: I3da39a17fdf8ffc4cc7d48a3b0cb2e2d39b020db
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>