intel/compiler: fix mesh urb write regression
Right now even the simplest mesh test (func.mesh.basic.mesh from crucible) fails like this:
ASSERT: Scalar MESH validation failed!
load_payload(16) vgrf11+0.0:F, vgrf8:D
../../src/intel/compiler/brw_fs_validate.cpp:61: inst->dst.offset / REG_SIZE + regs_written(inst) <= alloc.sizes[inst->dst.nr]
Because we try to load 8 regs with LOAD_PAYLOAD in SIMD16 mode.
Fixes: 349a040f ("intel/fs: Make logical URB write instructions more like other logical instructions")