drm/i915/psr: Warn for erroneous enabling of both PSR1 and PSR2.

Depending whether PSR1 or PSR2 was configured, we print a warning if the
corresponding control mmio indicated PSR was erroneously enabled. As
Chris pointed out, it makes more sense to check for both the mmio's
since we expect neither PSR1 nor PSR2 to be enabled when psr_activate() is
called.

v2: Read PSR2 control register only on supported platforms (Rodrigo)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180626090522.17682-1-dhinakaran.pandiyan@intel.com