glsl: Initialize lower_ubo_reference_visitor members in constructor.

glsl: Initialize lower_ubo_reference_visitor members in constructor.

Fix defects reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member buffer_access_type is not
initialized in this constructor nor in any functions that it
calls.
uninit_member: Non-static class member uniform_block is not
initialized in this constructor nor in any functions that it
calls.
uninit_member: Non-static class member progress is not initialized
in this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7120>