drm/msm: add DRM_IOCTL_MSM_RMFB2

Add new remove framebuffer ioctl that simply unreferences the
given framebuffer instead of triggering a shutdown of the CRTC
if the buffer is in active use. This allows the user space to
proactively unref the buffer without triggering an unwanted
shutdown.

Change-Id: Iac06985d069989b28affcf620d4e3feba6d07644
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>
Signed-off-by: Yunyun Cao <yunyunc@codeaurora.org>