testdisplay: test all modes if no option is provided

testdisplay: test all modes if no option is provided

Previously, when called without any arguments, the application required
user input to finish.  However, testdisplay is ran by "make check", and
it is not a good idea to run a program that requires user input in "make
check". So we change the default behavior to something that doesn't
require user input. You can still access the previous behavior using the
"-m" argument.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>