gallium/winsys/kms: Fix double refcount when importing from prime FD (v2)

gallium/winsys/kms: Fix double refcount when importing from prime FD (v2)

Currently the code creates a display target struct with refcount field
initialized to 1 and then the caller again increments it, leading to
a leaked reference. Let's remove the unnecessary increment.

v2: Split from one big patch into four fixing one thing at a time.

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