drm/amd/display: Add pixel_clock to amd_pp_display_configuration

This commit adds the pixel_clock field to the display config
struct so that power management (DPM) can use it.

We currently don't have a proper bandwidth calculation on old
GPUs with DCE 6-10 because dce_calcs only supports DCE 11+.
So the power management (DPM) on these GPUs may need to make
ad-hoc decisions for display based on the pixel clock.

Also rename sym_clock to pixel_clock in dm_pp_single_disp_config
to avoid confusion with other code where the sym_clock refers to
the DisplayPort symbol clock.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>