broadcom/vc5: Use SETMSF to handle discards.

A bit of spec text suggested that (like vc4) condition codes should be
used for discards, and the simulator was fine with it, but the 7268
disagrees and you have to use SETMSF instead or the color comes through.
Fixes glsl-fs-discard-01 and many of the interpolation-with-clipping
tests.

src/broadcom/compiler/v3d_compiler.h

@@ -458,9 +458,6 @@ struct v3d_compile {
         /* Fragment shader payload regs. */
         struct qreg payload_w, payload_w_centroid, payload_z;
 
-        /** boolean (~0 -> true) if the fragment has been discarded. */
-        struct qreg discard;
-
         uint8_t vattr_sizes[V3D_MAX_VS_INPUTS];
         uint32_t num_vpm_writes;