msm: mdss: Correct mixer enumeration for sdm630

For sdm630, LM0 and LM2 layer-mixers are present.
LM1 interface mixer is removed. So, enumerate the
LM2 mixer correctly.

Change-Id: Iceff8f215ca34e95556368a13a9b0c8aeb7c2ef3
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>