disp: msm: sde: Fix open method in sde_reg_fops and sde_off_fops

The open method for the debug fops are supposed to extract
the appropriate register base for the corresponding
hardware blocks, which are later used for the register
read and writes for the associated hardware block. But
the current open method being used points to some random
register base which can lead to fatal issues when performed
reads and writes to registers. Hence implemented a new
open method to extract the appropriate register base of the
various mdp hardware blocks. This patch also fixes
the valid sub range check against the corresponding
hardware blocks instead of checking across all the hardware
blocks.

Change-Id: I2a08fae74b3cbd31f2931c3b89b7308feb4b84e8
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>