samples: drivers: display: consolidate testcase.yaml

Consolidate the testcase.yaml definition for the display sample where
possible to use a single testcase for all shields. This won't work for
all testcases, but helps avoid the pattern where a new testcase using
"platform-allow" is added for every shield/board combination that needs
to be run with this sample.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>