st/mesa: fix the texture format in st_context_teximage

st/mesa: fix the texture format in st_context_teximage

Commit 1a339b6c71ebab6e1a64f05b2e133022d3bbcd15 made
st_ChooseTextureFormat map GL_RGBA with type GL_UNSIGNED_BYTE
to PIPE_FORMAT_A8B8G8R8_UNORM.

The image format for ARGB pixmaps is PIPE_FORMAT_B8G8R8A8_UNORM
however. This mismatch caused the texture to be recreated in
st_finalize_texture.

NOTE: This is a candidate for the 7.11 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=39209
Signed-off-by: Fredrik Höglund <fredrik@kde.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Brian Paul <brianp@vmware.com>