msm: mdss: add venus gdsc support in mdp

Venus gdsc has to be turned on before turning on
mdss gdsc due to hardware limitation for msm8996
v2 target. This change adds venus gdsc support
in mdp driver.

Change-Id: I2d6db6d43ac20d2e5196f5e31187246da38365d0
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>