modetest: document why no blob is created for linear gamma LUT

modetest: document why no blob is created for linear gamma LUT

As found and discussed in [MR 58] a blob is not created in the else arm
because adding the GAMMA_LUT property with a NULL/0 blob_id causes it
to be reset to a default linear / pass-thru gamma table.  The values
in the gamma_lut table might still be consumed in the legacy API path
below though, so it has to be initialized to a linear table.

[MR 58]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/58#note_466972

Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>