Limit the scaled clip to only clip at most dst_w/h pixels.
This avoids the problem with clip_scaled() not being able
to return negative values. Since new_src_w/h is now properly
bounded we can remove the clamp()s.

Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Testcase: igt/kms_selftest/drm_rect_clip_scaled_signed_vs_unsigned
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122175623.13565-3-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>