drm: ci: mt8173: Do not set IGT_FORCE_DRIVER to panfrost

Mediatek 8173 and 8183 SOCs have a separate display controller
and GPU with different drivers for each. For mt8173, the GPU
driver is powervr and the display driver is mediatek. In the
case of mt8183, the GPU driver is panfrost and the display
driver is mediatek. Setting IGT_FORCE_DRIVER to panfrost for
mt8173 is not the expected driver. So set mediatek for mt8173.

Support to test both GPU and display drivers for these ARM SOCs
will be added in the next patch series to increase test coverage.

Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207091831.660054-7-vignesh.raman@collabora.com