i965: downsize *64*PASSTHRU formats to equivalent *32*FLOAT formats on gen < 8

i965: downsize *64*PASSTHRU formats to equivalent *32*FLOAT formats on gen < 8

gen < 8 doesn't support *64*PASSTHRU formats when emitting
vertices. So in order to provide the equivalent functionality, we need
to downsize the format to equivalent *32*FLOAT, and in some cases
(R64G64B64 and R64G64B64A64) submit two 3DSTATE_VERTEX_ELEMENTS for
each vertex element.

Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>