i965/fs: Add a an optional source to the FS_OPCODE_FB_WRITE instruction

Previously, we were use the base_mrf parameter of fs_inst to store the MRF
location.  In preparation for doing FB writes from the GRF, we now also
allow you to set inst->base_mrf to -1 and provide a source register.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
4 files changed