glx: Don't use sRGB configs on llvmpipe with depth 16

Summary:
This is necessary to keep openQA working, which uses LLVMpipe as a
renderer on a Cirrus device that operates in depth 16.

LLVMpipe advertises 24/32 bit sRGB configurations on this setup, but
they cannot be presented.

CCBUG: 408594

Test Plan: Compile tested only.

Reviewers: fvogt, KWin, zzag

Reviewed By: fvogt, KWin, zzag

Subscribers: romangg, sbergeron, fvogt, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D22203