drm/i915/psr: Fix ALPM cap check for PSR2

While touching the code around this, I noticed that absence of ALPM
capability does not stop us from enabling PSR2. But, the spec
unambiguously states that ALPM is required for PSR2 and so does this
commit that introduced this code

drm/i915/psr: enable ALPM for psr2

    As per edp1.4 spec , alpm is required for psr2 operation as it's
    used for all psr2  main link power down management and alpm enable
    bit must be set for psr2 operation.

Cc: Jose Roberto de Souza <jose.souza@intel.com>
Cc: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Reviewed-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Reviewed-by: Tarun Vyas <tarun.vyas@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-6-dhinakaran.pandiyan@intel.com