mesa/st: Reuse st_choose_matching_format from st_choose_format().

We had this ad-hoc exact size matching for unsized internalformats,
but st_choose_matching_format() can do exactly what we want.  This
means, that, for example, we'll now prefer the matching ordering for
565/565_REV if the driver supports both orders.  We also pass
Unpack.SwapBytes through from ChooseTextureFormat so that we can hit
the memcpy path for 8888 formats when that flag is set.

Some interesting format choice changes from this (on softpipe):
intf/form/type        before            after
----------------------------------------------------
RGBA/RGBA/USHORT:     R8G8B8A8_UNORM -> RGBA_UNORM16
RGB/RGBA/8888:        X8B8G8R8_UNORM -> R8G8B8X8_UNORM
RGB/ABGR/8888_REV:    X8B8G8R8_UNORM -> R8G8B8X8_UNORM
RGBA/RGBA/5551:       B5G5R5A1_UNORM -> A1B5G5R5_UNORM
RGBA/RGBA/4444:       R8G8B8A8_UNORM -> A4B4G4R4_UNORM
RGBA/GL_RGBA/1010102: R8G8B8A8_UNORM -> A2B10G10R10_UNORM
DEPTH/DEPTH/UINT:     Z24X8          -> Z_UNORM32
DEPTH/DEPTH/USHORT:   Z24X8          -> Z_UNORM16

v2: Make sure that the baseformat still matches.  v1 would pick
    MESA_FORMAT_L16_UNORM for RED/LUMINANCE/SHORT, when we clearly
    want a red format.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>