meson: don't use build_by_default for specific gallium drivers

meson: don't use build_by_default for specific gallium drivers

Using build_by_default : false is convenient for dependencies that can
be pulled in by various diverse components of the build system, the
gallium hardware/software drivers and state trackers do not fit that
description. Instead, these should be guarded using the variable that tracks
whether that driver should be enabled.

This leaves a few helper libraries: trace, rbug, etc, and the generic
winsys bits as `build_by_default : false` because there are a large
number of gallium components that pull them in.

v2: - remove build_by_default from winsys convenience libs as well.
v3: - Always put drivers before winsys for consistency

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Tested-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v1)
Reviewed-by: Eric Anholt <eric@anholt.net>