msm: mdss: Allocate correct space for hist bin count in 64 bit

Histogram bin count variable was being allocated insufficient
space in compat ioctl for histogram read.
Correcting the space being allocated to the member variable
bin_cnt in histogram read.

Change-Id: I91111037ee5c73f8090d867b4924d3dc59b91d97
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>