drm/ci: add subset-1-gfx to LAVA_TAGS and adjust shards

drm/ci: add subset-1-gfx to LAVA_TAGS and adjust shards

The Collabora Lava farm added a tag called `subset-1-gfx` to half of
devices the graphics community use.

Lets use this tag so we don't occupy all the resources.

This is particular important because Mesa3D shares the resources with
DRM-CI and use them to do pre-merge tests, so it can block developers
from getting their patches merged.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Link: https://lore.kernel.org/r/20231024004525.169002-7-helen.koike@collabora.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>