radv,radeonsi,ac,nir: 20.2 backports

A bunch of commits listed in https://lists.freedesktop.org/archives/mesa-dev/2020-December/224775.html that are still not in staging/20.2

nir/lower_returns: Append missing phis' sources after "break" insertion had a conflict because of changes to nearby code.

radv: Fix budget calculations with large BAR. had a conflict because cf2eebdf is not in staging/20.2 and 9948ead3 (included in this MR) was incorrectly marked as merged into staging/20.2

radv: Fix exporting/importing multisample images. had a conflict because V_008F14_IMG_FMASK_* was renamed to V_008F14_IMG_NUM_FORMAT_FMASK_* in 20.3 but not in 20.2.

radeonsi: determine correctly if switching from normal launch to fast launch had a conflict because of changes to nearby code and the addition of a unlikely() that is not in staging/20.2.

ac: fix detection of Pro graphics and ac: Don't negate strstr return values in ac_query_gpu_info had conflicts because tabs were replaced with spaces.

ac: fix min/max_good_num_cu_per_sa on gfx10.3 with disabled SEs had a conflict because tabs were replaced with spaces and ac_print_gpu_info() switched from printf() to fprintf().