mesa/format_utils: Add a function to convert a mesa_format to an array format

mesa/format_utils: Add a function to convert a mesa_format to an array format

This commits adds the _mesa_format_to_array function that determines if the
given format can be represented as an array format and computes the array
format parameters. This is a direct helper function for using
_mesa_swizzle_and_convert

v2: Better documentation and commit message
v3: Fixed a potential segfault from an invalid endianness swizzle

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>