encore display panel: Kconfig-selectable default timings

Experimentation has shown that we want different refresh rates depending
on whether DSSCOMP is being used -- a higher rate to provide a smoother
experience if it's disabled, a lower rate to allow 720p video to be
downscaled in hardware if it's enabled.  While the panel timings are
adjustable from userspace, add a Kconfig option to change the default
timings used upon boot.
Add a new encore defconfig for CM11+

We want to use DSSCOMP on CM11+ builds, but this requires some changes
to the default display timings, which we now set via Kconfig options.