msm: mdss: Fix pipe number calculation for IGC

MDP hardware supports inverse gamma correction(IGC) on the source pipes.
Driver will program the LUT(Look up table) on the source pipe and
enable the IGC. Register exposes a bit mask which is equal to number of
pipes for which IGC LUT table needs to updated.
Source pipe enum values are not contiguous in nature which was resulting
in incorrect programming of IGC module.
This change fixes the issue by assigning bit mask position instead of
relying on the source pipe enum values.

Change-Id: I219cc484edd42fdd563dcb7535ceacb11e9e2d38
Signed-off-by: Ping Li <pingli@codeaurora.org>