drm/fb: small cleanup

Flip conditional to reduce indentation level of rest of fxn, and use
min/max to make the code clearer.

v2: surface_width -> surface_height typo

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>