drm/i915/dsi: Fix overflow issue in pclk parsing

Parsed divider p will overflow and is considered being valid in case
pll_ctl == 0.

Fix this by checking divider p before decreasing it. Also small improvement
is made by using fls() instead of custom loop.

v2: use fls() and check parsed divider

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250807042635.2491537-1-jouni.hogander@intel.com