gpu/drm: tegra: Fix build for Linux v6.8

Building the Tegra DRM driver with Linux v6.8 fails with the following
error ...

 drivers/gpu/drm/tegra/hdmi.c:623:25: error: implicit declaration of
 function ‘drm_eld_size’; did you mean ‘mm_cid_size’?
 [-Werror=implicit-function-declaration]
  623 |    size_t length = drm_eld_size(hdmi->output.connector.eld), i;
      |                    ^~~~~~~~~~~~

Commit ("8eb80946ab0c drm/edid: split out drm_eld.h from drm_edid.h")
added the header file 'drm/drm_eld.h' which now needs to be included to
build the Tegra DRM driver. Fix the build issue by using conftest to
detect the presence of the header 'drm/drm_eld.h' and include this
header if present.

Bug 4448428

Change-Id: Ifb17648d1dc721e1f0de3f864f4cc76450eea394
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/3027485
(cherry picked from commit 8d5286c379)
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/3055252
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>
GVS: Gerrit_Virtual_Submit <buildbot_gerritrpt@nvidia.com>

scripts/conftest/Makefile

@@ -182,6 +182,7 @@ NV_HEADER_PRESENCE_TESTS = \
   drm/dp/drm_dp_aux_bus.h \
   drm/dp/drm_dp_helper.h \
   drm/drm_dp_aux_bus.h \
+  drm/drm_eld.h \
   generated/autoconf.h \
   linux/kconfig.h \
   linux/iosys-map.h \