drm/amd/display: drop redundant memset() in get_available_dsc_slices()

get_available_dsc_slices() returns the number of indices set, and all of
the users of get_available_dsc_slices() don't cross the returned bound
when iterating over available_slices[]. So, the memset() in
get_available_dsc_slices() is redundant and can be dropped.

Fixes: 97bda032 ("drm/amd/display: Add DSC support for Navi (v2)")
Reported-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>