meson: don't require glx/egl/gbm with gallium drivers

The gallium drivers do not require a DRI loader. Drop the artificial
and unnecessary restriction.

Fixes: af9d2761 ("meson: build libmesa_gallium")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 9d105818)