gallium/winsys/kms: Look up the GEM handle after importing a prime FD

gallium/winsys/kms: Look up the GEM handle after importing a prime FD

drmPrimeHandleToFD() will return the same GEM handle every time the same
buffer is imported, even from a different prime FD. Since GEM handles
are not reference counted, we need to make sure that each GEM handle is
referenced only by one display target struct, by looking it up in
kms_sw->bo_list first and bumping the refcount of the found dt on hit
and falling back to creating a new dt only on miss.

v2: Split into separate function.
    Use helper function for lookup.

v3 [Emil Velikov]:
    Rename kms_sw_displaytarget_{lookup,find_and_ref} (Jordan)

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com> (v2)
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>