mesa: be more explicit about gallium-llvm dependency

mesa: be more explicit about gallium-llvm dependency

Historically mesa recipe had separate variables for drivers that
depended on gallium-llvm PACKAGECONFIG. Since that time we started
listing other dependencies explicitly (e.g. libclc). Drop the
intermediate variables and make the dependency on gallium-llvm more
explicit.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>