mesa: check constant before null check

For most drivers this if statement is always going to fail so check the constant value first.

Signed-off-by: Timothy Arceri <t_arceri@yahoo.com.au>
Reviewed-by: Brian Paul <brianp@vmware.com>
freedreno/a3xx: fix blend opcode

Seems the opcodes are slightly different from a2xx.  Resync headers and
move blend_func() helper into hw generation specific code.

Signed-off-by: Rob Clark <robclark@freedesktop.org>