freedreno/turnip: Update GRAS_LAYER_CNTL to GRAS_MAX_LAYER_INDEX

freedreno/turnip: Update GRAS_LAYER_CNTL to GRAS_MAX_LAYER_INDEX

After some experimentation, I believe that GRAS_LAYER_CNTL is
actually just a count register storing the number of layers in the
render target. While debugging cube_array geometry tests, I noticed
that the blob was setting an unknown 0x8 to LAYER_CNTL, so I checked
the value of LAYER_CNTL for various layer sizes:

1: LAYER_CNTL=0
2: LAYER_CNTL=1
3: LAYER_CNTL=2
4: LAYER_CNTL=3
9: LAYER_CNTL=8
256: LAYER_CNTL=255
2000: LAYER_CNTL=1999

Seems like this register just stores a count of the largest layer
that can be written to via gl_Layer. This commit updates the reg
docs, freedreno's gs implementation, and turnip's gs implementation.

Fixes dEQP-VK.geometry.layered.cube_array.*

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4541>