drm/i915: Cleanup fbdev fb setup

drm/i915: Cleanup fbdev fb setup

We use a mix of 'intel_fb' vs. 'ifbdev->fb' in the same function.
Both should be pointing at the same thing. Make things less
confusing by just getting existing fb from 'ifbdev->fb' at the
start and then sticking with the local 'fb' (renamed from the
'intel_fb') until the very end.

v2: rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240510102257.25096-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>