drm/i915/de: register wait function renames

Do some renames on the register wait functions for clarity and brevity:

intel_de_wait_for_register	-> intel_de_wait
intel_de_wait_for_register_fw	-> intel_de_wait_fw
__intel_de_wait_for_register	-> intel_de_wait_custom

In particular, it seemed odd to have a double-underscored function be
called in a number of places.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240320160123.2904609-1-jani.nikula@intel.com