Fix not checking return value of drmIoctl function call to map dumb buffer

Fix not checking return value of drmIoctl function call to map dumb buffer

in drm_fb_create_dumb, the return value of the drmIoctl function call
to map the dumb buffer was never checked, thus the following "if
(ret)" check was invalid as it was checking the previous return value
from the above drmModeAddFB call.

Signed-off-by: Chris Michael <cp.michael@samsung.com>