drm/omap: fix TILER on OMAP5

On OMAP5 it is not possible to use TILER buffer with CPU when caching or
write-combining is used. Doing so leads to errors from the memory
manager.

However, on OMAP4, write-combining works fine.

This patch adds platform specific data for the TILER, and a function
tiler_get_cpu_cache_flags() which can be used to get the caching mode to
be used.

Note that without write-combining the use of the TILER buffer with CPU
is unusably slow. It's still good to have it operational for testing
purposes.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>