etnaviv: Basic MSAA support
This MR brings back basic MSAA rendering support. Was tested with kmscube, the "Cube OpenGL ES 2.0 example" from qt5 and some deqp2/3 tests.
Stuff like texelFetch, texelFetch for MSAA and glSampleCoverage(..) is still WIP. RE is done and I started to write some nir lowerings.