tgsi/virgl: Handle TGSI_PROPERTY_SEPARABLE_PROGRAM only in virgl
Most drivers don't care about the property, and virgl should only handle it if the host supports it, so partially revertb6340305, i.e. keep the definition and set the property in virgl if the host can handle it.
Fixes: b6340305 tgsi: Add SEPARABLE_PROGRAM property
Regarding the first patch: @anholt @alyssa
virglrenderer patch: virgl/virglrenderer!846 (merged)