msm: vidc: Initial commit for VMEM driver

This commit adds initial support for the VMEM driver. VMEM is a new
hardware module that's part of upcoming MSM chipsets, and is intended to
be used as a pixel cache by Venus.

The VMEM driver enables the Venus driver to make use of the VMEM module by
providing APIs for memory allocation and deallocation.

Change-Id: If40bcf0cc27adbb4b0339e40e7e0226dfd3d54b9
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>