drm/amd/display: Check BIOS images before it is used

[ Upstream commit 8b0ddf19cca2a352b2a7e01d99d3ba949a99c84c ]

BIOS images may fail to load and null checks are added before they are
used.

This fixes 6 NULL_RETURNS issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>