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 ‘drm_intel_gem_bo_unreference’:
../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