radeonsi: Fix some warnings.

radeonsi: Fix some warnings.

../mesa/src/gallium/drivers/radeonsi/si_compute_blit.c: In function ‘si_clear_buffer’:
../mesa/src/gallium/drivers/radeonsi/si_compute_blit.c:195:11: warning: unused variable ‘clear_alignment’ [-Wunused-variable]
  unsigned clear_alignment = MIN2(clear_value_size, 4);
           ^~~~~~~~~~~~~~~
[23/60] Compiling C object 'src/gallium/drivers/radeonsi/3cdc30e@@radeonsi@sta/si_compute_prim_discard.c.o'.
../mesa/src/gallium/drivers/radeonsi/si_compute_prim_discard.c: In function ‘si_prepare_prim_discard_or_split_draw’:
../mesa/src/gallium/drivers/radeonsi/si_compute_prim_discard.c:1106:7: warning: unused variable ‘compute_has_space’ [-Wunused-variable]
  bool compute_has_space = sctx->ws->cs_check_space(cs, need_compute_dw, false);

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>