drm/amdgpu: Add default case in DVI mode validation

drm/amdgpu: Add default case in DVI mode validation

amdgpu_connector_dvi_mode_valid() assigns max_digital_pixel_clock_khz
based on connector_object_id using a switch statement that lacks a
default case.

In practice this code path should never be hit because the existing
cases already cover all digital connector types that this function is
used for. This is also legacy display code which is not used for new
hardware.

Add a default case returning MODE_BAD to make the switch exhaustive and
silence the static analyzer smatch error. The new branch is effectively
defensive and should never be reached during normal operation.

Fixes: 585b2f685c56 ("drm/amdgpu: Respect max pixel clock for HDMI and DVI-D (v2)")
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Timur Kristóf <timur.kristof@gmail.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>