msm: mdss: do not use mdp_lock for hist irq enable/disable

Calls to hist irq enable and disable are encompassed with a
spin_lock mdp_lock. This is not needed as all histogram irq
programming is independent of other MDP irq programming and
simultaneous access is restricted using hist intr lock. The
enable/disable irq calls made are further protected by one
mdss_lock spin_lock for preventing simultaneous access to
enable and disable of MDP irqs.

Change-Id: I2ec63e35fc3eca953e9ac21f7052c705656d899b
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>