drm: add assertion for checking null edid to drm_edid_block_valid

If raw_edid of drm_edid_block_vaild() is null, it will crash, so
checking in bad label is removed and instead assertion is added at
the top of the function.
The type of return for the function is bool, so it fixes to return
true and false instead of 1 and 0.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>