drm/amd: add missing fill of the array`s first element

In function fill_iram_v_2, the ram_table->bright_neg_gain`s
first element [0][0] seems to be missing. This change is just
to make the code a bit readable.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>