staging: sm750fb: change defintion of PANEL_PLL_CTRL multi-bit fields

Use more straight-forward definitions for multi-bit field of
PANEL_PLL_CTRL register and use open-coded implementation for register
manipulations.

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>