gallium/winsys/kms: Move display target handle lookup to separate function

gallium/winsys/kms: Move display target handle lookup to separate function

As a preparation to use the lookup in more than once place, move the
code that looks up given KMS/GEM handle to a separate function. This
change should not introduce any functional changes.

v2: Split into separate patch.
    Move lookup code into separate function.

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>