OMAPDSS: HDMI: Replace spinlock with mutex in hdmi_check_hpd_state

State change of HDMI PHY could potentially take many millisecs, we can do
better by protecting things in hdmi_set_phy_pwr() with a mutex rather than
a spin_lock_irqsave.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>