gfx: drv: Fix double free in page flip

gfx: drv: Fix double free in page flip

Reference counted object was accidentally freed using kfree without
checking reference count. That led to double free when kfree was called
with reference counting higher than 1. That happened often when HDMI
clone mode was enabled.

Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
CC: "Kannan, Arun" <arun.kannan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>