msm_fb: Handle histogram IRQ cleanly during suspend/resume

Currently, if the histogram disable IOCTL comes after panel
is turned off, we early return the IOCTL causing the interrupt
to be left enabled. This causes IPC issues during suspend. Fix
this issue by properly enabling/disabling the histogram interrupt
during suspend/resume scenario.

CRs-Fixed: 434491
Change-Id: I4d38776a43ea854aff3c10c8d9d5450af1d98c91
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>