drm/omap: fix issue w/ fb attached to multiple CRTCs

When the fb is detached from one CRTC/plane, paddr was set back to
zero.  But really we don't want to do this because the fb could still
be attached to other CRTC/plane(s).  This originally worked like this
to catch cases of freeing a pinned fb (but with the refcnt'ing this
should no longer be needed).  Also, there is checking in the GEM code
for freeing a pinned GEM object, so this extra level of checking is
redundant.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>