travis: meson: port gallium build combinations over

travis: meson: port gallium build combinations over

This commit adds a number of build combinations:

 - Gallium Drivers {SWR, RadeonSI, Others)
Each one has different LLVM requirements. Building SWR alone is twice
as slow as all other drivers combined.

 - Gallium ST Clover LLVM {5,6,7}
Because C++ API changes all the time. Analogous to above building
Clover takes as much time as building all other ST combined.

 - Gallium ST Others
Nouveau is used, instead of i915g since meson has explicit target
tracking. Meaning that a configure error is thrown if we use i915g
with say va, vdpau or others.

Note: LLVM prior to 5.0 is intentionally dropped. If needed we can add
that later.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>