They aren't going anywhere, and probing on DDC can cause the panel to
blank briefly, so read them up front and cache them for later queries.

v2: fix potential NULL derefs in intel_dp_get_edid_modes and
    intel_dp_get_edid (Jani)
    copy full EDID length, including extension blocks (Takashi)
    free EDID on teardown (Takashi)
v3: malloc a new EDID buffer that's big enough for the memcpy (Chris)
v4: change handling of NULL EDIDs, just preserve the NULL behavior
    across detects and mode list fetches rather than trying to re-fetch
    the EDID (Chris)
v5: be glad that Chris is around to remind me to hit C-x C-s before
    committing.

References: https://bugs.freedesktop.org/show_bug.cgi?id=46856Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>