drm/msm: a6xx: Refactor a6xx_sptprac_enable()

drm/msm: a6xx: Refactor a6xx_sptprac_enable()

A minor refactor to combine the subroutines for legacy a6xx GMUs under
a single check. This helps to avoid an unnecessary check and return
early from the subroutine for majority of a6xx gpus.

Also, document an intermediate unknown low power state which is not
exposed by the GMU firmware.

Signed-off-by: Akhil P Oommen <akhilpo@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/673364/
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>