i965/fs: Add a MAX_GRF_SIZE define and use it various places

i965/fs: Add a MAX_GRF_SIZE define and use it various places

Previously, we had a MAX_SAMPLER_MESSAGE_SIZE which we used instead.
However, some FB write messages can validly be longer than this so we need
something different.  Since MAX_SAMPLER_MESSAGE_SIZE is validly useful on
its own, we leave it alone and add a new MAX_GRF_SIZE that's big enough for
FB writes.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84539
Reviewed-by: Matt Turner <mattst88@gmail.com>