i965/vec4: pass the correct src_sz to emit_send at emit_untyped_atomic

i965/vec4: pass the correct src_sz to emit_send at emit_untyped_atomic

If the src is invalid, so src size is zero, the src_sz passed to emit
send should be zero too, instead of a default 1 if we are in a simd4x2
case. This can happens if using emit_untyped_atomic for an atomic
dec/inc.

v2: use the proper src_sz when calling emit_send, instead of just
    avoid loading src at emit_send if BAD_FILE (Francisco Jerez)

Reviewed-by: Francisco Jerez <currojerez@riseup.net>