disp: msm: dp: fix simulated hpd_irq handling for real monitors

A mode switch on a real monitor can be triggered by forcing the new
mode using debugfs and initiating an hpd_irq. Due to a missing
check for simulator state, the current driver calls the hpd_irq
callback for simulator even for a real monitor. This change adds
this check.

Change-Id: I13480eccd27eac2f9df3dd766d0445c0a5ea9b2c
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>