drm/i915/psr: Remove alpm from i915_psr

ALPM is a requirement and we don't need to keep it's cached, what
were done in commit 97c9de66
("drm/i915/psr: Fix ALPM cap check for PSR2") but the alpm was not
removed from i915_psr.

Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003205031.32474-7-jose.souza@intel.com