radeon: fix bgrx8/xrgb8 blits

radeon: fix bgrx8/xrgb8 blits

Since d21320f6258b2e1780a15c1ca718963d8a15ca18 the same txformat table entries
are used for "normal" texturing as well as for blits. However, I forgot to put
in an entry for the bgrx8 (le) and xrgb8 (be) formats - the normal texturing
path can't hit them because the radeon tex format chooser will never chose
them, but we get that format from the dri buffers (at least I assume we got
it from there). This caused lots of piglit regressions (and probably lots of
trouble outside piglit too).
This fixes bug https://bugs.freedesktop.org/show_bug.cgi?id=92900.

Tested-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 983614dbede7b94cba1bad9f3e8627fc5e14bb91)