drm/i915/bdw+: Move misc display IRQ handling to it own function

Just moving it to reduce the tabs and avoid break code lines.
No behavior changes intended here.

v2:
- Reading misc display IRQ outside of gen8_de_misc_irq_handler() as
other irq handlers (Dhinakaran)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730224753.14907-1-jose.souza@intel.com