drm/edid: add drm_edid_raw() to access the raw EDID data

Unfortunately, there are still plenty of interfaces around that require
a struct edid pointer, and it's impossible to change them all at
once. Add an accessor to the raw EDID data to help the transition.

While there are no such cases now, be defensive against raw EDID
extension count indicating bigger EDID than is actually allocated.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb55d0b580d556bf2b8e58070239657ac9cb4b2f.1656494768.git.jani.nikula@intel.com