drm/radeon/kms: suppress a build warning (unused variable)

drm/radeon/kms: suppress a build warning (unused variable)

At least 'make CONFIG_DEBUG_SECTION_MISMATCH=y' causes
drivers/gpu/drm/radeon/atombios_crtc.c: In function 'atombios_crtc_set_pll':
drivers/gpu/drm/radeon/atombios_crtc.c:684: warning: 'pll' may be used uninitialized in this function
which has the looks of a falso positive.

Add a default: case so that gcc rests assured that all possible pll_id's are covered.
Keep the present cases that fall through to the default one for self-documentation.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>