mesa: Pack gl_program_parameter struct

mesa: Pack gl_program_parameter struct

The gl_register_file doesn't need 16 bits, so shorten it and use the
extra room for 'Padded' (also mark it as a single bit).  This shrinks
the struct size from 32 bytes to 24 bytes.

See also 4794fbc86e3 ("mesa: reduce the size of gl_program_parameter")
that shrinked from 40 to 24 and later 7536af670b7 ("glsl: fix shader
cache for packed param list") that added `Padded`.

v2: Use just 5 bits for gl_register_file.  (Timothy)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>