msm: mdss: Replace enums with strings in panel dtsi

To make dtsi files more readable, enums or u32
values that referred to strings in the panel
dtsi files are replaced by strings.

Change-Id: I516358c7027fe9ed05541b56c2de47280a23baab
CRs-Fixed: 511801
Signed-off-by: Casey Piper <cpiper@codeaurora.org>