drm/tests: make sure drm_client_modeset tests are enabled

drm/tests: make sure drm_client_modeset tests are enabled

Default config for UML (x86_64) doesn't include any driver which
supports DRM_CLIENT_SELECTION, which makes drm_client_modeset disabled
(and correspondingly tests for that module are not executed too).

Enable DRM_VKMS and DRM_FBDEV_EMULATION in order to be able to run DRM
client modesetting tests.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://lore.kernel.org/r/20250821-drm-client-tests-v1-1-49e7212c744a@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>