i965/vec4: make offset() work in terms of a simd width and scalar components

i965/vec4: make offset() work in terms of a simd width and scalar components

So that it has the same semantics as the scalar backend implementation. The
helper will now take a simd width (which is always 8 in vec4 mode) and step
as many scalar components as specified by that width, respecting the size of
the scalar channels.

v2 (Curro):
  - Remove the assertion in offset(), byte_offset() has the same checks.
  - Use byte_offset() directly instead of add_byte_offset().
  - Make things more clear by explicitly including the vertical stride
    in the byte offset expression.

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