drm/i915/psr: Check if VBT says PSR can be enabled.

Driver features data block has a boolean flag for PSR, use this to decide
whether PSR should be enabled on a platform. The module parameter can
still be used to override this.

Note: The feature currently remains disabled by default for all platforms
irrespective of what VBT says.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180509003524.3199-1-dhinakaran.pandiyan@intel.com