meson: don't build gallium dri target if gallium is disabled

meson: don't build gallium dri target if gallium is disabled

Otherwise -Dgallium-drivers= will cause libmesa_gallium to be built and
the megadriver install script to attempt to install drivers without any
actual drivers being built.

fixes: 66f97f6640f5316b36177fd1053f0027eb6ec6cc ("meson: build radeonsi")
Reported-by: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Tested-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>