drm/fb-helper: Fix clipping when damage area spans a single scanline

When the damage area resulting from a dirty memory range spans a single
scanline, the width of the rectangle is calculated dynamically because it
may not coincide with the framebuffer width.
If the dirty range ends exactly at the end of the scanline, the `bit_end`
variable is incorrectly assigned a 0 value, which results in a bogus clip
rectangle where the x2 coordinate is 0. This prevents the dirty scanline
from being flushed to the hardware.
Change the calculation of the `bit_end` value to fix the x2 coordinate
value in the above edge case.

Fixes: ded74cafeea9 ("drm/fb-helper: Clip damage area horizontally")
Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260210173545.733937-1-flavra@baylibre.com

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