i965: Check for alpha channel just like in gen6+.

i965: Check for alpha channel just like in gen6+.

gen6+ uses _mesa_base_format_has_channel() to check for the alpha
channel, while gen4-5 use ctx->DrawBuffer->Visual.alphaBits. By using
_mesa_base_format_has_channel() here we keep the same behavior accross
all gen.

While initially both ways of checking the alpha channel seemed correct
to me, this change also seems to fix fbo-blending-formats piglit test on
gen4.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>