drm/i915/display: Use initial_fastset_check() to compute and apply the initial PSR state

Replace the previous approach to force compute the initial PSR state
after i915 take over from firmware by the better and recently added
initial_fastset_check() hook.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102221048.104294-1-jose.souza@intel.com