Change flags used for `drmPrimeHandleToFD` in broadcom drivers

This commit changes the calls drmPrimeHandleToFD to add the flag O_RDWR in the vc4 and v3d drivers. This is done so that if a user of the bo would like to mmap a bo created by these drivers, then we need to make sure that the FD is created with these flags, otherwise mmap will fail with a permission error.

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