drm/i915/display: convert modeset setup to struct drm_i915_private *i915

Pass struct drm_i915_private * instead of struct drm_device *, and
rename dev_priv to i915.

v2: Rebase

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617094817.3466584-2-jani.nikula@intel.com

drm/i915/display: split out hw state readout and sanitize

Split out the modeset hardware state readout and sanitize, or state
setup, to a separate file.

Do some drive-by checkpatch fixes while at it.

v2: Rebase

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220617094817.3466584-1-jani.nikula@intel.com