drm: fix warnings in DSC

Remove always false comparisons due to limited range of nfl_bpg_offset
and scale_increment_interval fields.
Warnings detected when compiling with W=1.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911084759.6946-1-benjamin.gaignard@st.com