drm/i915: Reorder intel_psr2_config_valid()

Future patches will bring PSR2 selective fetch configuration
validation but most of the configuration checks will be used for HW
tracking and selective fetch so the reoder was necessary.

Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200626010151.221388-2-jose.souza@intel.com