radeonsi/gfx9: fix si_get_buffer_from_descriptors for 48-bit pointers

This fixes:
GL45-CTS.pipeline_statistics_query_tests_ARB.functional_compute_shader_invocations

Cc: 18.0 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit 6d671078)