msm_fb: Handle multiple histogram reader errors during suspend/resume

If a histogram reader waits for completion in the do_histogram ioctl
and during this time the device is suspended, the complete for this
gets called from mdp_histogram_ctrl_all() in the panel_next_off()
sequence. This results in mgmt->hist not set to NULL. The subsequent
do_histogram ioctls after device resume fail due to multiple reader
failure condition since the histogram irq will be enabled now from
the same mdp_histogram_ctrl_all() during panel_next_on() sequence and
this by-passes mgmt->hist getting set to NULL that is present in
histogram_start() ioctl. Handle this scenario.

Change-Id: Ib0485da24c0f09a3f26768b75231903c8a2e0033
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>