drm/i915: Check LSPCON vendor OUI

Intel LSPCON chip is provided by 2 vendors:
- Megachips America (MCA)
- Parade technologies (Parade tech)

Its important to know the vendor of this chip, as the address to
write AVI infoframes is different for those two.

This patch reads the vendor OUI signature, and marks into LSPCON
encoder structure for future usages.

This patch also does a small re-arrangement of the code, by moving
lspcon mode change into probe function.

V2: Use dp->desc for OUI detection, dont add a helper for this
    (Ville)
V3: Rebase, Added r-b from Maarten
V4: Rebase
V5: Rebase
V6: Rebase
V7: Rebase
V8: Rebase
V9: Rebase
V10: Rebase

Cc: Imre Deak <imre.deak@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1539325394-20788-4-git-send-email-shashank.sharma@intel.com