tests/core_hotunplug: Select health checks for initially detected chipset

As soon as we first time open a drm device to be exercised, we identify
its chipset and perform some basic device specific checks.  We also
install a filter that matches the device bus address to make sure we will
exercise still one and the same device after each driver unbind-rebind /
device unplug-rediscover cycle.

However, each time before we select device specific health checks, whether
before or after a driver unbind-rebind / device unplug-rediscover
sequence, we identify the device chipset again.  Besides being more
expensive, that approach could also result in using different set of
health checks should device filters not work as expected.

Store detected chipset type at test start and always use that information
instead of identifying the chipset now and again.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>