drm/i915/psr: Do not override PSR2 sink support

Sink can support our PSR2 requirements but userspace can request
a resolution that PSR2 hardware do not support, in this case it
was overwritten the PSR2 sink support.
Adding another flag here, this way if requested resolution changed
to a value that PSR2 hardware can handle, PSR2 can be enabled.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328223046.16125-6-jose.souza@intel.com