drm/i915: Add transcoder restriction to PSR2

According to PSR2_CTL definition in BSpec there is only one instance
of PSR2_CTL. Platforms gen < 12 with EDP transcoder only support PSR2
on TRANSCODER_EDP while on TGL PSR2 is only supported by
TRANSCODER_A.

Since BDW PSR is allowed on any port, but we need to restrict by
transcoder.

v8: Renamed _psr2_supported_in_trans() to psr2_supported() (Lucas)

v9: Renamed psr2_supported() to transcoder_has_psr2() (Ville)

BSpec: 7713
BSpec: 20584
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820223325.27490-2-jose.souza@intel.com