msm_fb: Fail fb2 open if called during suspend

At present, if msm_fb_open() for fb2 is called during suspend,
we increment the ref_count for fb2 and return success without
turning on the writeback pipeline. This causes the WFD client
think that fb2 is successfully opened whereas fb2_open should
not successful during suspend. Hence, return error if msm_fb_open()
is called for fb2 during suspend.

CRs-fixed: 475310
Change-Id: I73167a120a64ac18c5789ccc9cf7faad14b50b94
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>