assembler: Factor out the destination register validation
assembler: Factor out the destination register validation

The goal is to use brw_set_dest(), so let's start by validating the
register we have before generating the opcode.

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

yipdw/intel-gpu-tools,ChrisCummins/intel-gpu-tools,mv0/intel-gpu-tools,erikarn/intel-gpu-tools,rib/intel-gpu-tools,chenxianqin/intel-gpu-tools,mv0/intel-gpu-tools,erikarn/intel-gpu-tools,rib/intel-gpu-tools,tiagovignatti/intel-gpu-tools,yipdw/intel-gpu-tools,yipdw/intel-gpu-tools,mv0/intel-gpu-tools,dlespiau/checkmate-test-igt,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,rib/intel-gpu-tools,dlespiau/checkmate-test-igt,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,ChrisCummins/intel-gpu-tools,rib/intel-gpu-tools,tiagovignatti/intel-gpu-tools,dlespiau/checkmate-test-igt,ChrisCummins/intel-gpu-tools,erikarn/intel-gpu-tools,erikarn/intel-gpu-tools,chenxianqin/intel-gpu-tools,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools,ChrisCummins/intel-gpu-tools