drm/i915/psr: Don't avoid PSR when PSR2 conditions are not met.

We can still use PSR1 when PSR2 conditions are not met.

So, let's split the check in a way that we make sure has_psr
gets set independently of PSR2 criteria.

v2: Duh! Handle proper return to avoid breaking PSR2.
v3: (DK):
	- better name for psr2 conditions check function
	- Don't remove FIXME block and psr2.support check.
	- Add a debug message to show us what PSR or PSR2 is
	  getting enabled now we have ways to enabled PSR on
	  PSR2 panels.
	- s/PSR2 disabled/PSR2 not enabled

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180227212913.14083-2-rodrigo.vivi@intel.com