drm/i915: Fix eDP DPCD aux max backlight calculations

Max backlight value for the panel was being calculated using byte
count i.e. 0xffff if 2 bytes are supported for backlight brightness
and 0xff if 1 byte is supported. However, EDP_PWMGEN_BIT_COUNT
determines the number of active control bits used for the brightness
setting. Thus, even if the panel uses 2 byte setting, it might not use
all the control bits. Thus, max backlight should be set based on the
value of EDP_PWMGEN_BIT_COUNT instead of assuming 65535 or 255.

Additionally, EDP_PWMGEN_BIT_COUNT was being updated based on the VBT
frequency which results in a different max backlight value. Thus,
setting of EDP_PWMGEN_BIT_COUNT is moved to setup phase instead of
enable so that max backlight can be calculated correctly. Only the
frequency divider is set during the enable phase using the value of
EDP_PWMGEN_BIT_COUNT.

This is based off the original patch series from Furquan Shaikh
<furquan@google.com>:

https://patchwork.freedesktop.org/patch/317255/?series=62326&rev=3

Changes since original patch:
* Remove unused intel_dp variable in intel_dp_aux_setup_backlight()
* Fix checkpatch issues
* Make sure that we rewrite the pwmgen bit count whenever we bring the
  panel out of D3 mode

v2 by Jani:
* rebase
* fix readb return value check

Cc: Furquan Shaikh <furquan@google.com>
Tested-by: AceLan Kao <acelan.kao@canonical.com>
Tested-by: Perry Yuan <pyuan@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200116211623.53799-2-lyude@redhat.com

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