dumbfb: pass O_RDWR to drmPrimeHandleToFD()
dumbfb: pass O_RDWR to drmPrimeHandleToFD()

This enables write access to the dumb fb with mmap().

Signed-off-by: Tomi Valkeinen <e1ca4dbb8be1acaf20734fecd2da10ed1d46a9bb@ti.com>

tomba/kmsxx,tomba/kmsxx,tomba/kmsxx,tomba/kmsxx