msm: mdss: Correct ARGC read write for all DSPP

Currently ARGC read write is done for DSPP0 irrespective of
dspp_num, as the dspp_num corresponding to the block provided
is not calculated.

Added the calculation of the dspp_num to get correct address
of ARGC registers.

Change-Id: Id787e76debb26d23d75d3cdb859a697b6f0016f5
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>