drm/amd/display: Use kvfree() to free coeff in build_regamma()

Use kvfree() instead of kfree() to free coeff in build_regamma()
because the memory is allocated with kvzalloc().

Fixes: e752058b

 ("drm/amd/display: Optimize gamma calculations")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>