etnaviv: Report correct number of vertex buffers

etnaviv: Report correct number of vertex buffers

The GCnano has only 4 vertex buffers instead of 16. This information
can be extracted from the GPU status registers and is already stored
in screen->specs.stream_count. Use PIPE_CAP_MAX_VERTEX_BUFFERS to
report this information and permit u_vbuf to reorganize the shaders
to fit.

This fixes the following dEQP on GCnano:
dEQP-GLES2.functional.shaders.conversions.vector_combine.float_float_float_float_to_vec4_vertex

This fixes all the other dEQP-GLES2.functional.shaders.conversions.*
which used to fail on GCnano.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3241>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3241>