xf86drm: rework drmGetDevices()

Do a once off memory allocation for each drmDevice.

This allows us to ease the error handling and simplify the
de-duplication loop. As part of this we need to rework drmFreeDevice()
such so that it frees the relevant hunks, rather than leaving that to
the caller.

Some memory stats from the drmdevice test

before: 22 allocs, 22 frees, 66,922 bytes allocated
after:   9 allocs, 9 frees, 66,436 bytes allocated

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>