drm/i915/display: add namespace to intel_prepare_reset

Rename intel_prepare_reset to intel_display_prepare_reset, so it's clear
from gt/ that we are calling out the display code.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106225531.920641-1-lucas.demarchi@intel.com