staging: drm/omap: avoid aquiring mutex in atomic context (v2)

omap_gem_roll() could be called by fbcon in atomic context or when
struct_mutext is held.  Avoid aquiring mutex (deadlock), or calling
tiler_pin() (which itself is not safe for atomic context) in these
cases.

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