gma500: Fix uninitialized variable and style issues

The return variable of psb_gtt_pin() may be used
uninitialized. Also fixed some coding style issues.

Signed-off-by: Andre Bartke <andre.bartke@gmail.com>
[Reapplied by hand due to other changes]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>