drm/msm: fix unbalanced DRM framebuffer init/destroy

drm/msm: fix unbalanced DRM framebuffer init/destroy

When msm_framebuffer_init() fails before calling drm_framebuffer_init(),
drm_framebuffer_cleanup() [called in msm_framebuffer_destroy()]
is still being called even though drm_framebuffer_init() was not
called for that buffer. Thus a NULL pointer derefencing:

[  247.529691] Unable to handle kernel NULL pointer dereference at virtual address 0000027c
...
[  247.563996] PC is at __mutex_lock_slowpath+0x94/0x3a8
...
[  247.823025] [<c07c3c78>] (__mutex_lock_slowpath) from [<c07c3fac>] (mutex_lock+0x20/0x3c)
[  247.831186] [<c07c3fac>] (mutex_lock) from [<c0347cf0>] (drm_framebuffer_cleanup+0x18/0x38)
[  247.839520] [<c0347cf0>] (drm_framebuffer_cleanup) from [<c036d138>] (msm_framebuffer_destroy+0x48/0x100)
[  247.849066] [<c036d138>] (msm_framebuffer_destroy) from [<c036d580>] (msm_framebuffer_init+0x1e8/0x228)
[  247.858439] [<c036d580>] (msm_framebuffer_init) from [<c036d630>] (msm_framebuffer_create+0x70/0x134)
[  247.867642] [<c036d630>] (msm_framebuffer_create) from [<c03493ec>] (internal_framebuffer_create+0x67c/0x7b4)
[  247.877537] [<c03493ec>] (internal_framebuffer_create) from [<c034ce34>] (drm_mode_addfb2+0x20/0x98)
[  247.886650] [<c034ce34>] (drm_mode_addfb2) from [<c034071c>] (drm_ioctl+0x240/0x420)
[  247.894378] [<c034071c>] (drm_ioctl) from [<c011df7c>] (do_vfs_ioctl+0x4e4/0x5a4)
...

Signed-off-by: Stephane Viau <sviau@codeaurora.org>
[plus initialize msm_fb to NULL to -Rob]
Signed-off-by: Rob Clark <robdclark@gmail.com>