msm: mdss: extend debugfs support for dsi phy registers

Expose dsi phy registers as well, as part of the debugfs
interface in addition to the dsi controller registers.

Change-Id: I49c8723e9f650137990df31b099c67e3b08907a3
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>