drm/ci: mediatek: add tests for powervr gpu driver

For mediatek mt8173, the display driver is mediatek, while the
gpu driver is powervr. Currently, in drm-ci for mt8173, only the
display driver is tested. Add support in drm-ci to test powervr
driver for mt8173. Powervr driver was merged in linux kernel,
but there's no mediatek support yet. So disable the powervr:mt8173
job which uses powervr driver.

Also update the MAINTAINERS file to include xfails for powervr driver.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>