getGpuDmaBufUsageKb: Report usage in Kilobytes

getGpuDmabufUsageKb currently reports the total size of
dma-bufs attached by the GPU device from
/sys/kernel/dmabuf/buffer/<inode>/size nodes. However these
sysfs nodes report the buffer size in bytes.

Convert the value returned by getGpuDmaBufUsageKb to KB.

Bug: 193403146
Test: Manual
Change-Id: Ie17382794f7e4db34ddadf75488d8f82de435ae0