tests/pm_rpm: refactor the modesetting code

tests/pm_rpm: refactor the modesetting code

Refactor the code so the params for the modesets are always stored and
don't have to be recomputed every time. With this, we can get rid of
our FB caching structures, and we now also allow the subtests that do
special modesets to reuse the parameters used on the default modesets,
just changing whatever they want.

The biggest win here is that the test case subfunctions get smaller
and simpler to read and write.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>