i965/vec4/scalarize_df: support more swizzles via vstride=0

i965/vec4/scalarize_df: support more swizzles via vstride=0

By exploiting gen7's hardware decompression bug with vstride=0 we gain the
capacity to support additional swizzle combinations.

This also fixes ZW writes from X/Y channels like in:

mov r2.z:df r0.xxxx:df

Because DF regions use 2-wide rows with a vstride of 2, the region generated
for the source would be r0<2,2,1>.xyxy:DF, which is equivalent to r0.xxzz, so
we end up writing r0.z in r2.z instead of r0.x. Using a vertical stride of 0
in these cases we get to replicate the XX swizzle and write what we want.

Reviewed-by: Matt Turner <mattst88@gmail.com>