drm/msm: Add API support for getting GPU hang timeout

Userspace needs to know the GPU timeout value to support Khronos robust
GPU timeout extension. The timeout value is returned to the user in
millisecond resolution.

Change-Id: Iba2ff43fce6d21da240356b392afa7a6e7a618ad
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>