drm/i915/psr: Check for SET_POWER_CAPABLE bit at PSR init time.

By moving the check from psr_compute_config() to psr_init_dpcd(), we get
to set the dev_priv->psr.sink_support flag only when the panel is
capable of changing power state. An additional benefit is that the check
will be performed only at init time instead of every atomic_check.

This should change the psr_basic IGT failures on HSW to skips.

v2: Return early when SET_POWER_CAPABLE bit is 0 (Jose)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106217
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106346


Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-2-dhinakaran.pandiyan@intel.com