drm/i915/psr: Allow PSR2 to be enabled when debugfs asks

For now PSR2 is still disabled by default for all platforms but is
our intention to let debugfs to enable it for debug and tests
proporses, so intel_psr2_enabled() that is also used by debugfs to
decide if PSR2 is going to be enabled needs to take in consideration
the debug field.

v2: Using the switch/case that intel_psr2_enabled() already had to
handle this(DK)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190117205548.28378-1-jose.souza@intel.com