gpu/drm: delete same check in if condition

In function drm_bridge_connector_get_modes_edid, drm_edid_is_valid
will check weather (!edid), no need to check again in the if
branch.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102030736.3833-1-bernard@vivo.com