clk: mdss: Remove pll support for all targets except msm8996

As part of 3.18 upgrade, remove support pll support for all
other targets except msm8996.

Change-Id: Idc778ccba25ce22ad7e418c45f2bd8d21ccb95e8
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>
Signed-off-by: Narendra Muppalla <narendram@codeaurora.org>