gallium: add scalar isa shader cap

gallium: add scalar isa shader cap

v1 -> v2:
 - nv30 is _NOT_ scalar as suggested by Ilia Mirkin.
 - Change from a screen cap to a shader cap as suggested
   by Eric Anholt.
 - radeonsi is scalar as suggested by Marek Olšák.
 - Change missing ones to be scalar.

v2 -> v3:
 - r600 prefers vec4 as suggested by Marek Olšák.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>