util: factor out and optimize C8 SMPTE color LUT

util: factor out and optimize C8 SMPTE color LUT

The color LUT for the SMPTE pattern in indexed mode contains 22 entries,
although only 13 are non-unique.

Reduce the size of the color LUT by dropping duplicate entries, so it
can be reused for formats supporting e.g. 16 colors.  Rename the
function util_smpte_c8_gamma() to util_smpte_fill_lut(), and its first
parameter size to ncolors, to match their actual use.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
---
v5:
  - Add Reviewed-by,

v4:
  - Rename util_smpte_index_gamma() to util_smpte_fill_lut(), and its
    first parameter from size to ncolors,
  - Move smpte_color_lut[] down,
  - Kill FILL_COLOR() macro,
  - Add and use EXPAND_COLOR() macro,

v3:
  - Add Acked-by,

v2:
  - Factor out smpte color LUT.