i965: Split merge_inputs and clear_buffers.

i965: Split merge_inputs and clear_buffers.

The merge_inputs function handles that part that changes when the
inputs change. The clear_buffers function triggers when we may need
a new upload. Thus the merge_inputs can be limited to be once
per brw_draw_prims.

v2: Move declaration of attribute index into the for scope.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/308>