radeonsi/sqtt: rework pm4.reg_va_low_idx

radeonsi/sqtt: rework pm4.reg_va_low_idx

The initial logic was to remember the place were SPI_SHADER_PGM_LO_*
are written, then assume that we can get the register offset because
the sequence would always be:

   PKT3_SET_SH_REG
   SPI_SHADER_PGM_LO_* register offset
   VA low 32 bits value <- reg_va_low_idx

The problem is that this sequence isn't guaranteed, for instance we
can get this instead:

   0   c0067600 |
   1   00000046 |
   2   003ffffd | SPI_SHADER_PGM_RSRC3_VS
   3   00000020 | SPI_SHADER_LATE_ALLOC_VS
   4 * 00002080 | SPI_SHADER_PGM_LO_VS
   5   00000080 | SPI_SHADER_PGM_HI_VS

So the assert in si_state_draw.cpp would fail as well as the VA
update logic.

So instead remember which the SPI_SHADER_PGM_LO_* offset, and the low
32 bits of the VA in si_update_shaders.

Fixes: 8034a71430b ("radeonsi/sqtt: re-export shaders in a single bo")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26774>
(cherry picked from commit b55a2065e03e0f033217b1b58a0c18e3a5e86136)