msm_fb: display: fix dsi_irq race condition

There has race condition that mipi_dsi_disable_irq_nosync() is
called from isr context at one core while mipi_dsi_irq_enable()
is called from thread context at other core. Serialize
mipi_dsi_disable_irq_nosync() with mipi_dsi_irq_enable() by
putting mipi_dsi_disable_irq_nosync() inside scope of
spin_lock(&dsi_mdp_lock).

CRs-fixed: 345837
Change-Id: Ibd2ff3194890b671983142fdb7b5a62cc009cae2
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>