drm/display: Fix building with GCC 15

drm/display: Fix building with GCC 15

GCC 15 enables -Werror=unterminated-string-initialization by default.
This results in the following build error

drivers/gpu/drm/display/drm_dp_dual_mode_helper.c: In function ‘is_hdmi_adaptor’:
drivers/gpu/drm/display/drm_dp_dual_mode_helper.c:164:17: error: initializer-string for array of
 ‘char’ is too long [-Werror=unterminated-string-initialization]
  164 |                 "DP-HDMI ADAPTOR\x04";
      |                 ^~~~~~~~~~~~~~~~~~~~~

After discussion with Ville, the fix was to increase the size of
dp_dual_mode_hdmi_id array by one, so that it can accommodate the NULL
line character. This should let us build the kernel with GCC 15.

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241002092311.942822-1-brahmajit.xyz@gmail.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>