pvr: Correctly validate PBE accum format

pvr: Correctly validate PBE accum format

Fixes: dEQP-VK.pipeline.monolithic.blend.format.e5b9g9r9_ufloat_pack32
  .states
  .color_sas_1mda_add_alpha_1msa_sas_rsub
    -color_1mda_cc_min_alpha_cc_cc_min
    -color_1mcc_1mca_rsub_alpha_1mca_1mda_min
    -color_sc_1mda_sub_alpha_sa_cc_min

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reported-by: James Glanville <james.glanville@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22795>