drm/rp1: depends on, instead of select, MFD_RP1

drm/rp1: depends on, instead of select, MFD_RP1

According to kconfig-language.txt [1], select should be used only for
"non-visible symbols ... and for symbols with no dependencies". Since
MFD_RP1 both is visible and has a dependency, "select" should not be
used and "depends on" should be used instead.

In particular, this fixes the build of this kernel tree on NixOS, where
its kernel config system will try to answer 'M' to as many config as
possible.

[1] https://www.kernel.org/doc/html/latest/kbuild/kconfig-language.html

Signed-off-by: Ratchanan Srirattanamet <peathot@hotmail.com>