gallium: Add support for 5551 with the 1-bit field in the low bit.

gallium: Add support for 5551 with the 1-bit field in the low bit.

This is how VC4 stores 5551 textures, which we need to support for
GL_OES_required_internalformat.

v2: Extend commit message, fix svga driver build, add BE ordering from
    Roland.
v3: Rebase on PIPE_FORMAT_R10G10B10X2_UNORM addition.

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v2)