omap: fix compiler warning

When compiling with linaro toolchain version 4.6.2 got this warning.

  CC     omap_drm.lo
omap_drm.c: In function 'omap_bo_new_impl':
omap_drm.c:139:6: warning: 'bo' may be used uninitialized in this function [-Wuninitialized]

This patch initialize bo to NULL avoiding the warning.

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Signed-off-by: Rob Clark <rob@ti.com>