virgl: Lower binding start into buffer indices

virgl: Lower binding start into buffer indices

This requires that we limit the number of max combinded SSBOs to 31,
otherwisewe shaders that use SSBO binding points with higher values
will break on the host.

Fixes CTS:
  KHR-GL43.shader_storage_buffer_object.basic-atomic-case1
  KHR-GL43.shader_storage_buffer_object.basic-atomic-case2
  KHR-GL43.shader_storage_buffer_object.advanced-indirectAddressing-case2
  KHR-GL43.shader_storage_buffer_object.advanced-usage-case1
  KHR-GL43.shader_storage_buffer_object.advanced-usage-sync
  KHR-GL43.shader_storage_buffer_object.advanced-matrix

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21586>