drm/i915: split out i915_switcheroo.[ch] from i915_drv.c

Split out code related to vga switcheroo register/unregister and state
handling from i915_drv.c into new i915_switcheroo.[ch] files.

It's a bit difficult to draw the line how much to move to the new file
from i915_drv.c, but it seemed to me keeping i915_suspend_switcheroo()
and i915_resume_switcheroo() in place was the cleanest.

No functional changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20191004122019.12009-2-jani.nikula@intel.com