drm/vc4: Fix leak of HDMI EDID

drm/vc4: Fix leak of HDMI EDID

We don't keep a pointer to it around anywhere, so it's our job to free
it.

Cc: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://github.com/anholt/linux/issues/101
Fixes: b5ab5d6e3cb4 ("drm/vc4: Add KMS support for Raspberry Pi.")
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170808205605.4432-1-eric@anholt.net
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>