drm/amd/display: Add check for granularity in dml ceil/floor helpers

drm/amd/display: Add check for granularity in dml ceil/floor helpers

[Why]
Wrapper functions for dcn_bw_ceil2() and dcn_bw_floor2()
should check for granularity is non zero to avoid assert and
divide-by-zero error in dcn_bw_ functions.

[How]
Add check for granularity 0.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>