llvmpipe: Double number of SSBOs

llvmpipe doesn't define atomics, and therefore, gallium uses half of the available SSBOs as atomic buffers. When virgl runs with llvmpipe as GLES host we also report no atomic buffers to the guest so as to lower atomics to SSBOs to work around ARB_shader_counter_ops not being available on GLES. With the current value of 16 SSBOs in llvmpipe, only eight SSBOs and eight atomic shader buffers are reported and with virglrenderer not reporting atomics these numbers will be reduced to four, being below the required minimum for ARB_compute_shaders.

By doubling the number of available SSBOs in llvmpipe, this limitation is worked around and using llvmpipe as GLES host for virgl in the CI allows testing ARB_compute_shaders and ARB_shader_counter_ops properly.