st/mesa: honour sized internal formats in st_choose_format (v2)

st/mesa: honour sized internal formats in st_choose_format (v2)

The bitcasting which is possible with shader images (and texture views?)
requires that when the user specifies a sized internal format for a
texture, we really allocate that format. To this end:

(1) find_exact_format should ignore sized internal formats and

(2) some of the entries in the mapping table corresponding to sized
    internal formats are reordered to use an RGBA format instead of
    a BGRA one.

This fixes arb_shader_image_load_store-bitcast in the (work in progress)
ARB_shader_image_load_store implementation for radeonsi.

v2: don't change the mapping of GL_RGB10: the change caused a regression
    because it preferred a format with an alpha channel, and GL_RGB10
    is not among the supported formats for shader images

Reviewed-by: Marek Olšák <marek.olsak@amd.com>