gallium: Add PIPE_CAP_GLSL_TESS_LEVELS_AS_INPUTS
In the Intel backend, it makes the most sense to treat gl_TessLevelInner and gl_TessLevelOuter as ordinary shader inputs. For Radeon, it makes more sense to treat them as system values which get special handling.
We already have a compiler option for this, but the Iris driver will need a capability bit so we can set it appropriately.