i965: Generalize wm_key->compute_sample_id to wm_key->multisample_fbo.

i965: Generalize wm_key->compute_sample_id to wm_key->multisample_fbo.

I'm going to need a key entry meaning "we have a multisample FBO,
and multisampling is enabled" in an upcoming patch.  This is basically
wm_key->compute_sample_id, except that it also checks that the SAMPLE_ID
system value is read.

The only use of wm_key->compute_sample_id is in emit_sampleid_setup(),
which is only called when handling the SAMPLE_ID system value.  So we
can just eliminate the check and generalize the field.

v2: Also update the Vulkan driver.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>