drm/i915: split intel_modeset_init() pre/post gem init

Turn current intel_modeset_init() to a pre-gem init function, and add a
new intel_modeset_init() function and move all post-gem modeset init
there, in the correct layer. No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5f4603f2c0216dba980338f00e0bfa791b526231.1599056955.git.jani.nikula@intel.com