meta: Use _mesa_unpack_format_to_base_format() to handle integer formats

meta: Use _mesa_unpack_format_to_base_format() to handle integer formats

Replace a call to mesa_base_tex_format() that handles only internal
formats with a call to the new _mesa_unpack_format_to_base_format()
function that handles allowed unpack formats and does not care for
internal formats at all.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>