drm/i915/tgl: Guard and warn if more than one eDP panel is present

On TGL+ it's possible to have PSR1 enabled in other ports besides DDIA.
PSR2 is still limited to DDIA. However currently we handle only one
instance of PSR struct. Lets guard intel_psr_init_dpcd() against
multiple eDP panels and warn about it.

v2: Reword commit message to be TGL+ only and with the info where
PSR1/PSR2 are supported (Lucas)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@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/20190823082055.5992-6-lucas.demarchi@intel.com