drm/gma500: Fix potential NULL pointer dereference

fb is being dereferenced before it is null checked, hence there
is a potential null pointer dereference.

Fix this by moving the pointer dereference after fb has been
properly null checked at line 74: if (!fb)

Addresses-Coverity-ID: 1470169 ("Dereference before null check")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625121844.GA12466@embeddedor.com