drm/tilcdc: Clean up LCDC functional clock rate setting code

Clean up LCDC functional clock rate setting code.

The LCDC functional clock is set by two functions: mode_set_nofb() and
cpufreq_transition().

When tilcdc_crtc_mode_set_nofb() is called in atomic commit phase the
drm atomic helpers have taken all the necessary drm locks and turned
off the crtc, while tilcdc_commit() is keeping LCDC powered on. For
mode_set_nofb() just a simple clock setting function without any
locking or power management code is enough. The new tilcdc_crtc_set_clk()
is implemented for that purpose.

cpufreq_transition() on the other hand is called from outside DRM and
it needs to take the necessary locks and turn off the CRTC while
keeping the LCDC powered. The reimplemented tilcdc_crtc_update_clk()
is for that purpose and it uses the new tilcdc_crtc_set_clk() to
actually set the clock.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>