i965/fs: Add gen6 register spilling support.

i965/fs: Add gen6 register spilling support.

Most of this is code movement to get the scratch space allocated in a
shared location.  Other than that, the only real changes are that the
old oword block messages now operate on oword-aligned areas (with new
messages for unaligned access, which we don't do), and that the
caching control is in the SFID part of the descriptor instead of
message control.

Fixes glsl-fs-convolution-1.