drm/i915/psr: Enable PSR1 on gen-9+ HW

We have new tests and fixes in place since the feature was last
disabled. Try again for gen-9+ hardware and enable only PSR1 by default as
a first step.
v2: Remove typo fix and comment improvements (Rodrigo)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Jose Roberto de Souza <jose.souza@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: commit 2ee7dc49 ("drm/i915: disable PSR by default on HSW/BDW")
References: commit dcb2e993 ("Revert "drm/i915: Enable PSR by default on Valleyview and Cherryview."")
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Tested-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928061117.12394-1-dhinakaran.pandiyan@intel.com