modetest: add SMPTE pattern support for C[124] formats

modetest: add SMPTE pattern support for C[124] formats

Add support for drawing the SMPTE pattern in buffers using a
color-indexed frame buffer formats with two, four, or sixteen colors.

Note that this still uses 256 as the CLUT size, as
DRM_IOCTL_MODE_SETGAMMA enforces that the size matches against the
(fixed) gamma size, while the CLUT size depends on the format.

Move clearing the color LUT entries from util_smpte_index_gamma() to its
caller, as only the caller knows how many entries there really are
(currently DRM always assumes 256 entries).

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:
  - Add missing C[12] to oneline-summary,
  - Do not remove memset() of full lut, else some entries may stay
    uninitialized,

v3:
  - Add Acked-by,

v2:
  - Split off changes to tests/modetest/modetest.c,
  - Add C1 and C2 support.

The linuxdoc comments say userspace can query the gamma size:

 * drm_mode_gamma_set_ioctl - set the gamma table
 *
 * Set the gamma table of a CRTC to the one passed in by the user. Userspace can
 * inquire the required gamma table size through drm_mode_gamma_get_ioctl.

 * drm_mode_gamma_get_ioctl - get the gamma table
 *
 * Copy the current gamma table into the storage provided. This also provides
 * the gamma table size the driver expects, which can be used to size the
 * allocated storage.

but the code doesn't seem to support that in an easy way (like setting
red/green/blue to NULL on input, retrieving gamma_size on output), only
by providing big enough buffers for red/green/blue, and looping over
gamma_size until -EINVAL is no longer returned.