freedreno/a6xx: support layered framebuffers in blitter_clear

To achieve this, I added the support for gl_Layer in vertex shader, however I'm not enabling GL_AMD_vertex_shader_layer since amd_vertex_shader_layer-layered-depth-texture-render fails at the moment (due to an issue with clearing arrayed depth buffers - not due to any issue with gl_Layer).

Clear is done with one instanced draw call, where the layer to clear is controlled by gl_Layer. Same as how util_blitter_clear does this.

Fixes test:
gl-3.2-layered-rendering-clear-color-all-types 2d_multisample_array single_level