drm/i915/display/adlp: Allow PSR2 to be enabled

With all the recent fixes PSR2 is properly working in Alderlake-P but
due to some issues that don't have software workarounds it will not be
supported in display steppings older than B0.

Even with this patch PSR2 will no be enabled by default in ADL-P, it
still requires enable_psr2_sel_fetch to be set to true, what some
of our tests does.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
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/20210930001409.254817-7-jose.souza@intel.com