drm/vc4: remove unused render_wait variable

smatch reports
drivers/gpu/drm/vc4/vc4_irq.c:60:1: warning: symbol
  'render_wait' was not declared. Should it be static?

This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230406151203.1953812-1-trix@redhat.com