intel: move declarations to top in drm_intel_gem_bo_unreference()

intel: move declarations to top in drm_intel_gem_bo_unreference()

Fixes the following warning:

    [65/74] Compiling C object intel/libdrm_intel.so.1.0.0.p/intel_bufmgr_gem.c.o
    ../intel/intel_bufmgr_gem.c: In function â\80\98drm_intel_gem_bo_unreferenceâ\80\99:
    ../intel/intel_bufmgr_gem.c:1388:9: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
     1388 |         drm_intel_bufmgr_gem *bufmgr_gem =
          |         ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>