drm: Restore the NULL check for drm_gem_object_put()

Some users want to pass NULL to drm_gem_object_put(), but those using
__drm_gem_object_put() did not. Compromise, have both and let the
compiler sort it out.

drm_gem_fb_destroy() calls drm_gem_object_put() with NULL obj causing:
[   11.584209] BUG: kernel NULL pointer dereference, address: 0000000000000000
[   11.584213] #PF: supervisor write access in kernel mode
[   11.584215] #PF: error_code(0x0002) - not-present page
[   11.584216] PGD 0 P4D 0
[   11.584220] Oops: 0002 [#1] SMP NOPTI
[   11.584223] CPU: 7 PID: 1571 Comm: gnome-shell Tainted: G            E     5.7.0-rc1-1-default+ #27
[   11.584225] Hardware name: Micro-Star International Co., Ltd. MS-7A31/X370 XPOWER GAMING TITANIUM (MS-7A31), BIOS 1.MR 12/03/2019
[   11.584237] RIP: 0010:drm_gem_fb_destroy+0x28/0x70 [drm_kms_helper]
<snip>
[   11.584256] Call Trace:
[   11.584279]  drm_mode_rmfb+0x189/0x1c0 [drm]
[   11.584299]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
[   11.584314]  drm_ioctl_kernel+0xaa/0xf0 [drm]
[   11.584329]  drm_ioctl+0x1ff/0x3b0 [drm]
[   11.584347]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
[   11.584421]  amdgpu_drm_ioctl+0x49/0x80 [amdgpu]
[   11.584427]  ksys_ioctl+0x87/0xc0
[   11.584430]  __x64_sys_ioctl+0x16/0x20
[   11.584434]  do_syscall_64+0x5f/0x240
[   11.584438]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   11.584440] RIP: 0033:0x7f0ef80f7227

Reported-by: Nirmoy Das <nirmoy.das@amd.com>
Fixes: b5d25074 ("drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Nirmoy Das <nirmoy.das@amd.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Christian König <christian.koenig@amd.com>.
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200520142347.29060-1-chris@chris-wilson.co.uk