meson: give dri- and gallium-drivers separate vars

meson: give dri- and gallium-drivers separate vars

Variables are cheap, and there's little reason for the dri and gallium
drivers to work on the same variable for the driver list. So let's split
these in two separate lists instead.

This makes it easier to inspect these after-the fact, for instance
for generating a summary of build-settings.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>