drm/msm: Implement shared virtual memory ioctl

Shared virtual memory allows the CPU and GPU to use
the same virtual address for a buffer. This change
implements an ioctl to allow creation of SVM buffer
objects.

Change-Id: I0d929a2e37a9eeef948dc2a37250c1eb9adf6fc7
Signed-off-by: Sushmita Susheelendra <ssusheel@codeaurora.org>