softpipe: don't use 3-component formats

softpipe: don't use 3-component formats

Mesa and gallium don't have a complete set of matching 3-component
texture formats.  For example, 8-bit sRGB unorm.  To fully support
the GL_ARB_copy_image extension we need to have support for all of
these formats: RGB8_UNORM, RGB8_SNORM, RGB8_SRGB, RGB8_UINT, and
RGB8_SINT using the same component order.  Since we don't have that,
disable the 3-component formats for now.

v2: Simplify 3-component format check, per Marek.
Also check that target != PIPE_BUFFER.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>